A nitrogen balance study was done to determine the protein requirement of healthy young women and to evaluate the impact of the menstrual cycle on nitrogen utilization. Six healthy young women were fed a defined formula diet with progressively decreasing levels of egg white proteins: 4 mg nitrogen/basal kcal for 42 days, 3 mg nitrogen/basal kcal for 28 days and 2.5 mg nitrogen/basal kcal for 22 days. Dietary energy was fixed at about 38 kcal/kg body weight and exercise was standardized. Excreta, menstrual fluids, and integumentary losses were collected. A statistically significant (P<.001) biphasic cycle in urinary nitrogen excretion was found in every subject. Linear regression analysis predicted nitrogen requirement to be 73 ± 20 mg nitrogen/kg body weight. When expressed per kilogram of body weight, there appears to be no significant difference in protein requirement between women and men. The urinary nitrogen cycle, however, was found to be unique to women, implying a hormonal regulation of nitrogen utilization. The failure to account for this cycle in women could lead to erroneous estimates of nitrogen requirement or large coefficients of variation.