High glycemic index (GI) carbohydrates may increase brain serotonin, which in turn acts to alleviate premenstrual symptoms, because, although the main determinant of brain serotonin concentration is a high plasma ratio of tryptophan to other large neutral amino acids, a high-GI diet has been shown to increase this ratio. In this observational cross-sectional study, we investigated associations between dietary GI and other dietary carbohydrates and premenstrual symptoms. Subjects were 640 female Japanese dietetic students 18-22 y of age. Dietary carbohydrates were assessed using a validated, self-administered, comprehensive diet history questionnaire. Menstrual cycle symptoms were assessed using the retrospective version of the Moos Menstrual Distress Questionnaire (MDQ). Independent associations of dietary GI and glycemic load and intake of available carbohydrate and dietary fiber with the MDQ total score and subscale scores (pain, concentration, behavioral change, autonomic reactions, water retention, and negative affect) in the premenstrual phase (expressed as percentages relative to those in the intermenstrual phase) were examined. Dietary GI was independently inversely associated with total MDQ score in the premenstrual phase (P for trend = 0.02). Dietary GI also showed independent and inverse associations with several MDQ subscale scores in the premenstrual phase, including concentration, autonomic reactions, and water retention (P for trend < 0.05). Conversely, dietary glycemic load and intake of available carbohydrate and dietary fiber were not associated with any of the MDQ scores in the premenstrual phase. Dietary GI was independently associated with decreased premenstrual symptoms in a group of young Japanese women.