The effect of long-term timolol treatment on heart size after myocardial infarction was evaluated by X-ray in a double-blind study including 241 patients (placebo 126, timolol 115). The follow-up period was 12 months. The timolol-treated patients showed a small but significant increase in heart size from baseline in contrast to a decrease in the placebo group. These differences may be caused by timolol-induced bradycardia and a compensatory increase in end-diastolic volume. The timolol-related increase in heart size was observed only in patients with normal and borderline heart size. In patients with cardiomegaly, the increase in heart size was similar in both groups. After re-infarction, heart size increased in the placebo group and remained unchanged in the timolol group.