The study protocol was reviewed in February 1978 and recommended for approval by the policy-data monitoring board and ad hoc members. The protocol was approved by the Director of NHLBI in March 1978. Recruitment started on June 19, 1978, and ended in October 1980. A total of 3,837 patients were randomized. Units which participated in the trial included 32 clinical centers, an EKG center, a central laboratory, a coordinating center, a 1-hour ambulatory ECG center, a 24-hour ambulatory EKG center, and an EKG tape quality control center.