The study has a case-control design. Data collected from the Group Health Cooperative (GHC) computerized files were used to identify potential cases: all treated hypertensive patients, aged 30 to 79 years, were eligible as cases if, according to WHO criteria, they presented with an incident, fatal or non-fatal MI. A random sample of members listed in the GHC enrollment files served as the source of potential controls. Review of the outpatient medical records ensured that all study subjects met the same entrance criteria. The effort also secured information about blood pressures, duration of hypertension, and past medical history. A telephone interview provided information about other potential confounders, including smoking, diet, and physical activity. The GHC computerized pharmacy records, a database of all prescriptions filled by enrollees, served as the primary source of information about the use of calcium-channel blockers. Frequency matching controlled for the potential confounding effects of age and year of presentation, and data analysis involved logistic regression.