Cross-sectional studies suggest that women who have irregular menstrual cycles and hyperandrogenism may be at increased risk for cardiovascular disease (CVD). However, prospective data are lacking on the relationship between menstrual cycle irregularity and subsequent CVD risk. The objective of this study was to assess prospectively the risk for coronary heart disease (CHD) and stroke associated with a history of irregular menstrual cycles. The study design was a prospective cohort study of 82,439 female nurses who provided information in 1982 on prior menstrual regularity (at ages 20-35 yr) and were followed through 1996 for cardiovascular events. Incident reports of nonfatal myocardial infarction, fatal CHD, and nonfatal and fatal stroke were made. Medical records were reviewed for confirmation. During 14 yr (1,155,915 person-yr) of follow-up, there were 1417 incident cases of CHD and 838 incident cases of stroke, including 471 cases of ischemic stroke. Compared with women reporting a history of very regular menstrual cycles, women reporting usually irregular or very irregular cycles had an increased risk for nonfatal or fatal CHD [age-adjusted relative risks (RR), 1.25 and 1.67, respectively; 95% confidence intervals (CI), 1.07-1.47 and 1.35-2.06, respectively]. Increased risks for CHD associated with prior cycle irregularity remained significant after adjustment for body mass index and several potential confounders. There was a nonsignificant increase in overall stroke risk (RR, 1.30; 95% CI = 0.97-1.74) and in ischemic stroke risk (RR, 1.40; 95% CI = 0.97-2.04) associated with very irregular cycles. Menstrual cycle irregularity may be a marker of metabolic abnormalities predisposing to increased risk for CVD.