OBJECTIVE: To measure adherence to statins by patients treated for secondary prevention after myocardial infarction and to estimate the effect of adherence on outcome. METHODS: We used a cohort design in the population of Tayside, Scotland. Patients who experienced their first MI between January 1990 and November 1995 were identified from hospital discharge data. We used two outcomes: mortality from any cause; hospitalization for recurrent MI. Adherence to statins was calculated as the number of days for which statins were supplied divided by the total number of days in the study for each patient. Results were adjusted for age, sex, deprivation (as measured by the Carstairs code), serum cholesterol level, diabetes mellitus, cardiovascular drugs and other hospitalization using a Cox regression model. RESULTS: Of 5590 patients enrolled in the cohort 1299 (23.2%) died during the follow-up period and 717 (12.8%) experienced. at least one further MI. Only 7.7% of patients used statins and, in comparison with non-users these patients had more cardiovascular risk factors. Compared to those not using statins, the adjusted relative risk of mortality (95% CI) by quintiles of adherence was 0.65 (0.24-1.80) for the worst adherence quintile, 0.46 (0.06-3.43) for the second, 1.03 (0.37-2.88) for the third, 0.19 (0.03-1.37) for the fourth, and 0.20 (0.09-0.47) for the best adherence quintile. The adjusted relative risks of readmission by quintiles of adherence were 0.65 (0.24-1.79) for the worst adherence quintile, 0.47 (0.06-3.51) for the second, 1.05 (0.37-2.94) for the third, 0.20 (0.03-1.41) for the fourth, and 0.21 (0.09-0.48) for the best adherence quintile. CONCLUSIONS: Statins were used infrequently and use was a marker of cardiovascular risk. Despite such confounding by indication, good adherence to treatment was associated with lower risks of further MI and lower mortality.