162 patients discharged from hospital after mycardial infarction were randomly allocated to two groups, one received alprenolol 400 mg daily and the other served as the control. The period of follow-up was two years and all other treatment given was standardized. The two groups did not differ with respect to risk factors for myocardial infarction, the course of the acute infarct or treatment during follow-up. After two years one patient in the group treated with alprenolol had died suddenly as compared to nine in the control group. During the same period four fresh infarcts had occurred in the alprenolol group compared to 15 in the control group. Both these differences were statistically significant. Only four patients were obliged to discontinue beta-blocker treatment because of suspected side-effects. Long-term post-infarction treatment with beta-blockers appears to be an effective form of secondary therapy without serious side-effects.