Historical data from 26 638 20-to 40-year women were used to study the association between obesity and menstrual abnormalities including evidence of infertility. It was found that women with evidence of anovulatory cycles, ie, irregular cycles greater than 36 days, and hirsutism, were more than 30 lb (13.6 kg) heavier than women with no menstrual abnormalities after adjusting for height and age. The percentage of women with evidence of anovulatory cycles was 2.6 per cent for women less than 20 per cent overweight, 4.0 per cent for women 20-49 per cent overweight, 5.8 per cent for women 50-74 per cent overweight., and 8.4 per cent for women more than 74 per cent overweight Women with a single menstrual abnormality including cycles greater than 36 days, irregular cycles, virile hair growth with facial hair, or heavy flow were also significantly heavier than women with normal values for these factors. A longer duration of obesity was associated with facial hair. Another analysis found that teenage obesity was greater for never-pregnant married women than for previously pregnant married women and for women having ovarian surgery for polycystic ovaries than for women having ovarian surgery for other reasons. This also supports an association of obesity with anovulatory cycles. These findings showing evidence of abnormal ovulation, menstrual abnormalities and excess hair growth in obese women may be explained by the recent studies of others demonstrating an association between obesity and hormonal imbalances.