The relation of self-reported high blood pressure to the subsequent development of coronary heart disease and stroke was examined in a cohort of 119,963 women, aged 30-55 years, who were initially free from cardiovascular disease. Participants in the Nurses' Health Study reported high blood pressure and other cardiovascular risk factors on baseline questionnaires mailed in 1976. During six years of follow-up, there were 308 incident cases of coronary heart disease (66 fatal and 242 nonfatal myocardial infarctions) and 175 strokes (50 fatal and 125 nonfatal). Fatal as well as nonfatal coronary heart disease and stroke were all significantly more frequent among the women who had reported high blood pressure. After adjusting simultaneously for age and other risk factors, the relative risks were 3.5 (95% confidence interval (Cl) 2.8-4.5) for total coronary heart disease and 2.6 (95% Cl 1.8-3.5) for total stroke. This association was evident at all levels of relative weight. The results emphasize the importance of high blood pressure as an independent predictor of coronary heart disease and stroke in middle-aged women and suggest that the increased risk occurs in both lean and obese women.