BACKGROUND: Higher risks of menstrual problems and infertility have been found in underweight and overweight women but evidence is inconsistent especially in relation to the effect of age of onset of obesity. OBJECTIVE: To determine whether body mass index (BMI) in adulthood or childhood affects the reproductive health of women. METHODS: Heights, weights (at 7, 11, 16, 23 and 33 y) and reproductive data were available for 5799 females in the 1958 British birth cohort study. Body mass index (BMI) was calculated as weight/height 2 . Age-speci®c cut-offs were used to de®ne overweight and obesity. Reproductive outcomes reported at age 33 included: menstrual problems (also reported at 16 y), hypertension in pregnancy and subfertility. RESULTS: Early menarcheal age was associated with higher risks of menstrual problems by 16 y but this relationship did not persist to 33 y. Obesity at 23 y and obesity at 7 y both independently increased the risk of menstrual problems by age 33 (OR 1.75, OR 1.59 respectively) after adjusting for other confounding factors. Obesity at 23 y increased the risk of hypertension in pregnancy (OR 2.37), after adjusting for confounders. Consistent with these ®ndings, obese women at 23 y were less likely to conceive within 12 months of unprotected intercourse after adjustment for confounders (RR 0.69). CONCLUSIONS: Overweight and obesity in early adulthood appears to increase the risk of menstrual problems, hypertension in pregnancy and subfertility. Other than menstrual problems, childhood body mass index had little impact on the reproductive health of women.