The influence of heart size on the effect of long term timolol treatment with regard to mortality and reinfarction after myocardial infarction was examined among 1881 patients randomised to either active or placebo treatment. The patients were followed for 12 to 33 months. At the baseline, heart size was determined by x-ray film in two projections: 1199 patients had normal heart size, 262 had borderline heart size, and 420 had enlarged hearts. The incidence of total cardiac death was three times greater in patients with enlarged hearts compared with patients with normal size hearts. The incidence of non-fatal reinfarctions, however, was independent of heart size at baseline. The timolol related reduction of total cardiac death compared with placebo was 40.7% in patients with normal heart size, 47.8% in patients with borderline heart size, and 38.2% in patients with enlarged hearts at baseline (intention to treat approach). The reduction of first non-fatal reinfarctions in the timolol group compared with placebo was, respectively, 31.7%, 41.2%, and 25.9%. Thus, timolol treatment appears to reduce cardiac death and non-fatal reinfarctions after myocardial infarction independent of heart size at baseline. Timolol treatment may be of special importance in patients with cardiomegaly, because of the very high incidence of cardiac mortality in this group of patients, and consequently a larger number of cardiac deaths may be prevented.