Luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol (E2), and progesterone (P) levels were followed in 22 healthy, normal-weight women (aged 19 to 30 years) for a control and a diet menstrual cycle. During the diet cycle, they lost weight on a high-carbohydrate, vegetarian, 1000-calorie diet. During the control cycle, luteal phase in 5 subjects failed to meet the criteria: length greater than or equal to 8 days and P maximum greater than or equal to 6 ng/ml; during the diet cycle, the number of subjects who failed to meet these criteria was 14 (chi-square test, P less than 0.02). No evidence of follicular phase disturbance was observed during the diet. Age and weight loss significantly changed parameters of the diet luteal phase: length and area under LH, FSH, E2, and P curves. Generally, hormone plasma concentrations during the luteal phase were lower the younger the age and the greater the weight loss.