OBJECTIVE: To investigate sexual activity as a trigger of myocardial infarction and the potential effect modification of physical fitness. DESIGN: A case-crossover study nested in the Stockholm Heart Epidemiology Programme (SHEEP). SETTING: Stockholm County from April 1993 to December 1994. PATIENTS: All patients with a first episode of non-fatal acute myocardial infarction admitted to coronary care units were eligible, and 699 patients participated in an interview. MAIN OUTCOME MEASURES: Relative risks with 95% confidence intervals. RESULTS: Only 1.3% of the patients without premonitory symptoms had sexual activity during two hours before the onset of myocardial infarction. The relative risk of myocardial infarction was 2.1 (95% confidence interval (CI) 0.7 to 6.5) during one hour after sexual activity, and the risk among patients with a sedentary life was 4.4 (95% CI 1.5 to 12.9). CONCLUSIONS: The increased risk of myocardial infarction after sexual activity and the further increase in risk among the less physically fit support the hypothesis of causal triggering by sexual activity. However, the absolute risk per hour is very low, and exposure is relatively infrequent. Thus having sex once a week only increases the annual risk of myocardial infarction slightly. Counselling should focus on encouraging patients to live a physically active life and not on abstaining from sexual activity.