We studied 41 nonvegetarian and 34 vegetarian premenopausal women whom we closely screened. The two groups were indistinguishable with respect to height, weight, body mass index, and menarche. The incidence of menstrual irregularity was 4.9% among nonvegetarians and 26.5% among vegetarians (P = 0.009). The vegetarian group consumed significantly greater amounts of polyunsaturated fatty acids, carbohydrates, vitamin B-6, and dietary fiber whereas the nonvegetarians reported greater intakes of saturated fatty acids, protein, cholesterol, caffeine and alcohol. Logistic-regression analyses showed that the probability of menstrual regularity among all study subjects (n = 75) was positively associated with increasing protein/kJ and increasing cholesterol/kJ intakes. The probability of being menstrually regular was negatively associated with increasing dietary fiber/kJ and increasing magnesium/kJ intakes. These results are consistent with the notion that menstrual regularity can be influenced by specific dietary nutrients that may have direct effects or exert their effects by modulating circulating sex steroid status.