Background Despite its health benefits, physical inactivity is pervasive, particularly among those living in lower-income urban communities. In such settings, neighborhood safety may impact willingness to be regularly physically active. We examined the association of perceived neighborhood safety with pedometer-determined physical activity and physical activity self-efficacy. Methods and Findings Participants were 1,180 predominantly racial/ethnic minority adults recruited from 12 urban low-income housing complexes in metropolitan Boston. Participants completed a 5-d pedometer data-collection protocol and self-reported their perceptions of neighborhood safety and self-efficacy (i.e., confidence in the ability to be physically active). Gender-stratified bivariate and multivariable random effects models were estimated to account for within-site clustering. Most participants reported feeling safe during the day, while just over one-third (36%) felt safe at night. We found no association between daytime safety reports and physical activity among both men and women. There was also no association between night-time safety reports and physical activity among men (p = 0.23) but women who reported feeling unsafe (versus safe) at night showed significantly fewer steps per day (4,302 versus 5,178, p = 0.01). Perceiving one's neighborhood as unsafe during the day was associated with significantly lower odds of having high physical activity self-efficacy among both men (OR 0.40, p = 0.01) and women (OR 0.68, p = 0.02). Conclusions Residing in a neighborhood that is perceived to be unsafe at night is a barrier to regular physical activity among individuals, especially women, living in urban low-income housing. Feeling unsafe may also diminish confidence in the ability to be more physically active. Both of these factors may limit the effectiveness of physical activity promotion strategies delivered in similar settings., Garry Bennett and colleagues measured exercise levels and obtained opinions on neighborhood safety. They concluded that residing in a neighborhood perceived to be unsafe at night is a barrier to regular physical activity., Editors' Summary Background. Nowadays we are all encouraged to do more physical activity, as it has been shown that inactivity increases the risk of many medical conditions, including obesity, heart attacks, and strokes. Inactive people die younger. Previous research has shown that people on a low income and those from ethnic or racial minorities have the lowest activity levels. There are, however, many barriers to being active. It has been claimed that people who live in neighborhoods that are unsafe face particular difficulties. They might want to walk, cycle, or take other forms of outdoor exercise near their home, but they fear they would be injured as a result of a violent attack. It is usually the poorest members of society who live in unsafe areas. It is also known that those poor people who belong to minority racial or ethnic groups are particularly likely to feel unsafe. Why Was This Study Done? The researchers who did this study wanted to find out whether people in a low-income urban area in the US considered themselves to be unsafe in their neighborhood, and how much physical activity they took part in. Their aim was to establish whether there was an association between the perceived safety level and the amount of activity taken, or if the two were unrelated. Other researchers have tried to look for such an association before, but they have usually relied on how much activity people say they remember taking, not on the actual measured amount. The results from such research have been very varied and inconclusive. What Did the Researchers Do and Find? Working in one low-income district of one US city (Boston), they found over 1,000 people to participate in their study, most of whom were from ethnic minorities. They asked them the question: “How safe do you feel walking alone in your neighborhood?” Response options included “safe,” “a little unsafe,” and “unsafe.” The same question was asked about walking alone in the daylight and walking alone after dark. The people in the study also agreed to wear a pedometer for five days. This instrument measures the number of steps that the wearer takes. It is thus a much more accurate way of finding out about activity levels than asking people how much activity they think they have engaged in. Four out of five people said they did feel safe during the day, but there was no association between daytime safety and physical activity. This was the case for both men and women. Two-thirds of the people in the study felt unsafe in the night-time. There was no association between perceived night-time safety and physical activity among men, but women who reported feeling unsafe at night took around 1,000 fewer steps per day than other women. That amounts to around 20% less physical activity. What Do These Findings Mean? Even the women who felt safe at night were only taking around 5,000 steps per day, around half of what the US Surgeon General recommends for good health. So all the women in the study would benefit from more physical activity. However, the much lower amount of activity of the women who felt unsafe does suggest that a perceived lack of safety is an important factor, which could increase the risks to their health. It is interesting that the association between perceived safety and activity was not found in men and only applied to night-time safety. However, the authors argue that their findings provide preliminary evidence that perceived low neighborhood safety may serve as a barrier to physical activity in low-income areas. They discuss in the article the need for further research. Additional Information. Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040306. • The UK National Health Service publishes the online NHS Direct Health Encyclopedia, which describes the benefits of exercise and gives recommendations • Information on exercise may also be found on MedlinePlus, a service of the US National Library of Medicine and the National Institutes of Health