An Institute Planning Committee met four times between October 1974 and April 1975 and developed a protocol, manual of operations, and data collection forms. Recruitment of patients began in June 1975, with the first patient randomized on July 2, 1975. Patients who were randomized had been seen at the AMIS Clinical Center for two initial visits and one baseline visit and were free of any reasons for exclusion, such as the current use of anticoagulants and a history of adverse reactions to aspirin. Patients took acetaminophen at times when they would normally take aspirin.