All admissions to a coronary care unit (CCU) in Copenhagen County were prospectively registered over a 10-year period, from 1979 to 1988, i.e. after the introduction of CCUs but before the era of intervention with thrombolytic or prophylactic medical treatment. The catchment area remained nearly constant throughout the study period; all patients with MI were admitted to the CCU regardless of age and concomitant diseases, and treatment and discharge policy of the department was unchanged. A total of 4176 MI admissions were registered. During the study period, the age-specific incidence of MI decreased in males above 50 years of age, but was virtually unchanged in females, increasing the proportion of women in the MI population from 26 to 33%. In-hospital and 30-day mortality was unchanged. The occurrence of atrial fibrillation during the admission increased significantly (from 11 to 18%), while the incidence of ventricular fibrillation, heart failure and pulmonary oedema was unchanged.