The primary hypothesis tested by the trial was that intensive blood pressure control under stepped care for five years could significantly reduce mortality compared with that under referred-care. Stepped-care was the method of treatment in HDFP clinics in which a diuretic was given initially and additional antihypertensive agents were added in a time-structured, stepwise fashion until goal blood pressure was achieved. Referred-care represented referral to private physicians and other community sources of care. Participating in this study were 14 clinical centers, a coordinating center, ECG center, central laboratory, and monitoring laboratory.