The positive association between maternal weight gain (MWG) during pregnancy and size at birth is well recognized. This study evaluated the effect of MWG on adolescent obesity. Data on 11,428 infants born between November 1974 and February 1976, was made available from the computerized records of the Jerusalem Perinatal Study, and matched to data at age 17 years obtained from the military draft medical examination records. The data were analyzed separately for females (n=4,383) and males (n=7,045). A logistic regression analysis was used to estimate the odds ratio for being overweight(>90th percentile for BMI) at age 17 years, adjusting for the confounding effect in the mother of prepregnant weight, body mass index (BMI), socioeconomic status, ethnic origin, education and occupation, and in the infant of birth order, birth weight and gestational age. Subjects whose mothers gained excessive weight during pregnancy (>16 kg; >90th percentile), were significantly (p <0.001) more likely to be overweight at 17 years of age compared with subjects whose mothers gained 11 to 16 kg, with odds ratios of 1.68 (95th confidence interval 1.32-2.15) for males, and 1.71(1.28-2.28) for females. The adjusted odds ratios, taking into account the above confounding variables, were 1.70 (1.31-2.20) and 1.74 (1.28-2.36) for males and females, respectively. These findings suggest that excessive MWG is associated with a higher risk of being overweight in late adolescence. These findings may reflect the effect of increased intrauterine nutrition or the influence of confounding extrauterine environmental factors.