To examine the benefits of thrombolytic therapy in diabetic patients with acute myocardial infarction a retrospective study of all diabetic and non-diabetic patients with acute myocardial infarction admitted to the coronary care unit of the General Hospital, Birmingham between January 1984 and December 1987 was made and findings compared to corresponding groups admitted between January 1990 and May 1992 when thrombolytic therapy was routine. In-hospital mortality and morbidity were assessed in 208 diabetic and 1029 non-diabetic patients with acute myocardial infarction admitted 1984 and 1987 and in 115 diabetic and 501 non-diabetic patients with myocardial infarction between January 1990 and May 1992. Following the introduction of thrombolytic therapy, there was a reduction in mortality among non-diabetic patients from 17% to 8.5%; p < 0.001 (observed reduction: 49%; 95% CI: 30-70%) and in the incidence of left ventricular failure (from 22% to 8%, p < 0.1 (observed reduction: 52%; 95% CI: 40-85.5%). Diabetic patients showed a reduction in mortality from 30% to 17%; p = 0.02 (observed reduction: 42%; 95% CI: 9.4-73.8%) and in the incidence of left ventricular failure from 39% to 21%; p < 0.01 (observed reduction: 45%; 95% CI: 20.3-72.5%). Thrombolytic therapy confers a major benefit on diabetic patients with acute myocardial infarction, although this group remains at a prognostic disadvantage compared to non-diabetic patients.