Objective Previous studies describing menses duration and heaviness of flow during the menopausal transition (MT) have been short in duration and limited to white women. We estimated the frequency of and risk factors for prolonged bleeding, spotting and heavy bleeding during the MT in an ethnically diverse population. Design Prospective community-based cohort study. Setting USA: southeastern Michigan, northern California and Los Angeles, California. Population A total of 1320 midlife women who participated in the Study of Women's Health Across the Nation (SWAN) Menstrual Calendar Substudy. Participants included African-American, white, Chinese, and Japanese women. Methods Women completed daily menstrual calendars from 1996 to 2006, and provided information on hormone therapy, smoking and physical activity. Annual measures included height and weight. Kaplan?Meier survival analysis and multivariable regression were used to analyse the data. Main outcome measures Menses of 10+ days, spotting of 6+ days, heavy bleeding of 3+ days. Results At least three occurrences of menses 10+ days was reported by 77.7% (95% confidence interval [95% CI] 56.7?93.2), of 6+ days of spotting by 66.8% (95% CI 55.2?78.0) and of 3+ days of heavy bleeding by 34.5% (95% CI 30.2?39.2) of women. Menses of 10+ days, 6+ days of spotting, and 3+ days of heavy bleeding were associated with MT stage, uterine fibroids, hormone use and ethnicity. Body mass index was associated with 3+ days of heavy bleeding. Conclusions These data provide clinicians and women with important information about the expected frequency of prolonged and heavy bleeding and spotting during the menopausal transition that may facilitate clinical decision making.