This study explores the clinical and endocrine implications of weight loss. The frequency of menstrual abnormalities was found to be greatly influenced by reduction in body weight. The occurrence of amenorrhea appeared to be proportional to the rate of weight loss, while regular cycles and luteal phase deficiency were inversely correlated to weight loss. Endocrine variations were noted. Significant decreases in luteinizing hormone and follicle stimulating hormone levels occurred only in amenorrheic patients with a weight loss higher than 20%. The estradiol level decreased in proportion to the reduction in body weight. Cortisolemia increased in all patients, with higher values apparent in amenorrheic women presenting with a weight loss higher than 20%. The prolactin values were normal except in amenorrheic patients who had lower levels. A reduction of insulin levels occurred in all groups, with a significant difference found only in amenorrheic women in Group C (weight loss higher than 20%). The data suggest that weight loss in young women causes alterations in the menstrual cycle mainly through two mechanisms involving both the hypothalamic-pituitary-adrenal axis and ovarian steroidogenesis together with estrogen catabolism.