The relation between degree of diastolic blood pressure reduction and mortality was examined among hypertensive persons in the Hypertension Detection and Follow-up Program. This program, conducted from 1973-1979, was a multicenter community-based trial, which followed 10,940 hypertensive participants for five years. The one-year annual visit was the first occasion on which change in blood pressure could be measured on all participants. During the subsequent four years of follow-up on 10,053 participants, 568 deaths occurred. With time-dependent life tables and time-dependent Cox life table regression analyses, the existence of a quadratic function which modeled the relation between diastolic blood pressure reduction and mortality was supported, even after adjusting for other risk factors. The minimum mortality hazard ratio, based on a particular model, occurred at a diastolic blood pressure reduction of 26.2 mmHg (standard error = 13.4) in the whole population and 10.0 mmHg (standard error = 5.3) in the baseline diastolic blood pressure stratum 90-104 mmHg. After this reduction, there was a small increase in the risk of death. There was no evidence of the quadratic function after fitting the same model with systolic blood pressure. Methodological issues involved in studying a particular degree of blood pressure reduction were considered. The confidence interval around the change corresponding to the minimum hazard ratio was wide, and the obtained blood pressure level should not be interpreted as a goal for treatment. Blood pressure reduction was attributed not only to pharmacologic therapy but also to regression to the mean, and to other factors unrelated to treatment.