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To investigate the effects of maternal smoking during pregnancy on the development of preeclampsia (PE).The study comprised two populations with singleton pregnancies, one collected from 1990 to 1994 at the Malmö University Hospital (Malmö series, n=14,510) and the other from the National Birth Registry of Sweden 1993 (National series, n=113,211). Women with PE (n=281 and n=2,865, respectively) were compared to those without PE, delivered in 1993 (n=2,811 and n=110,346, respectively). The subgroups of women who had PE associated with preterm birth (n=58 and n=693, respectively) were compared with the same control groups in both series.Multiple logistic regression analysis showed that, in comparison with non-smokers, moderate smokers (1-9 cigarettes per day) were characterized by a lower incidence of PE (odds ratio (OR) 0.4; 95% confidence interval (CI) 0.22-0.60), and PE associated with preterm birth (OR 0.1; 95% CI 0.01-0.67). The corresponding figures in the validating National series were also significantly lower for moderate smokers (OR 0.6; 95% CI 0.5-0.7 and OR 0.6; 95% CI 0.5-0.8, respectively).Moderate smoking during pregnancy seems to protect against the development of PE and PE associated with preterm birth. Nicotine might be the agent responsible for this protective effect.