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Background: The purpose of this study was to examine the change in accelerometer-derived daily physical activity (PA) and activity record-derived daily activities over a 10-year period in urban elementary schoolchildren in Japan. Methods: A total of 233 sixth-grade children (11-12 years old) in a same elementary school in Kyoto participated in the study (n = 125 and 108 in 1999 and 2009, respectively). The participant rate is 91.9 and 98.2 % in 1999 and 2009, respectively. The size and shape of the school district was not changed. The children were instructed to wear an accelerometer for 5 consecutive days of weekday and to keep minute-by-minute 24-h activity records with the assistance of their parents. The school-day scheduling was similar between 1999 and 2009 (29 and 27 sessions of 45-min class per week, respectively). Results: From 1999 to 2009, step counts considerably decreased (20,832 vs. 12,237 steps per day in boys and 16,087 vs. 10,748 steps per day in girls; P < 0.001) with concomitant significant decreases in total energy expenditure (P = 0.011), activity energy expenditure (P < 0.001), and physical activity level (P < 0.001). Time spent playing outdoors and walking to school were also significantly less in 2009 than 1999 (P < 0.001). Accelerometer-measured PA was significantly associated with the amount of time spent playing outdoors and walking to school. Conclusions: These results indicate that elementary schoolchildren in 2009 spend less time playing outdoors and walking to school, perform less PA, and take fewer steps than children of a decade ago. [ABSTRACT FROM AUTHOR]