The changes and determinants of blood pressure were examined in the Dormont High School (Pittsburgh, Pennsylvania) cohort of 86 men and 116 women with mean ages of 17 years during high school (1957-1963), 34 years at follow-up I (1977-1978), and 47 years at current follow-up II (1989-1990). Over the 30-year period, the subjects' mean systolic blood pressure changed relatively little, whereas the increase in mean diastolic blood pressure was significantly higher in men than in women (p < 0.01). Based on the criteria of diastolic blood pressure > or = 90 mmHg, and/or current use of antihypertensive medication, 18% developed hypertension. Compared with nonhypertensives, hypertensives had significantly higher baseline systolic blood pressure (p < 0.001); higher weight at all ages (p < 0.05); and gained more weight over the period (p < 0.01). By means of multivariate analyses, it was found that baseline systolic blood pressure, current weight, and weight gain were significantly and independently associated with current systolic blood pressure level and hypertension. These data indicate that initial systolic blood pressure level at adolescence, current weight, and weight gain are important determinants of risk of high blood pressure, and there is a further suggestion of sex and age differences in the critical period of risk.