The Determinants Study began in 1989, shortly after widespread acceptance of the circadian variation in Ml occurrence. The study was initiated to utilize state-of-the art epidemiologic techniques to determine the role of daily activities in the triggering of Ml. The investigators created a new epidemiologic technique (the case-crossover method), developed and tested a 24-page interview form, established a large study group of hospitals with trained interviewers, instituted quality control measures, and enrolled over 2,000 subjects.