Although birth rates to US women aged 25 and older have increased markedly over the last two decades, accurate estimates of the long-term weight gain associated with childbearing are not available for older mothers in the general population. We examined the effect of childbearing on weight change in 2547 white women aged 25-45 years who were initially weighed in the First National Health and Nutrition Examination Survey (1971-75) and who were reweighed an average of 10 years later. Linear and logistic regression estimates were adjusted for duration of follow-up, age, body mass index, initial parity, education, smoking, drinking, employment status, marital status, illness, physical activity, and dieting to lose weight. Compared to parous women who did not give birth during the study period, the mean excess weight gain was 1.6 kg (95% Confidence Limits, +/- 2.3 kg) for nulliparous women, and was 1.7 kg (+/- 1.1 kg), 1.7 kg (+/- 2.0 kg), and 2.2 kg (+/- 4.3 kg), for women having one, two and three live births, respectively. Among women who were nulliparous at baseline, those that had their live births during the study period gained similar amounts of weight to that of women who began childbearing before the beginning of the study. The risk of gaining more than 13 kg was increased by 40%-60%, and the risk of becoming overweight was increased by 60%-110% in women having live births during the study. We conclude that the average weight gain associated with childbearing after the age of 25 is quite modest in US white women. However, for some women who give birth after the age of 25 the risks of major weight gain and becoming overweight are increased in association wtih childbearing.