Nine normal young women of normal weight, aged 20-29 yr, who had regular menstrual cycles, dieted for 6 weeks (approximately 800-1000 kcal/day) and lost between 6 and 8 kg body wt. Half-hourly blood samples were taken from 1800-0530 h on two occasions before and after 1, 2, 3, 4, 5, and 6 weeks of dieting. In three women with anovulatory cycles the LH secretion pattern was not altered by dieting, but plasma estradiol levels decreased and reached menopausal concentrations during the final 2 weeks of dieting. In two of these three women the menstrual cycles were disrupted and regular cycles occurred only 3 and 6 months after dieting. Six women had regular ovulatory cycles. High progesterone values (greater than 3 ng/ml) were recorded in two cycles before the dieting period. While dieting, three women maintained ovulatory cycles and three women had no periovulatory hormone secretion pattern and/or a pattern characteristic of the luteal phase. No significant alterations of average LH concentrations and LH peak frequency developed. It is concluded that mild dieting does not suppress LH secretion in the manner found in anorexia nervosa or during total fasting. Dieting may interfere with gonadal steroid production, thus causing disturbances of the menstrual cycle. The effect described here may be responsible for the early onset of amenorrhea in patients with beginning anorexia nervosa.