To record the use of secondary prophylactic drugs in patients discharged from hospital having had a myocardial infarction. Prospective postal questionnaire survey of a random one in two sample of general practitioners in the region. The nine family practitioner committee areas within the Northern Regional Health Authority. Patients who had had a myocardial infarction and were discharged to their general practitioner. Whether beta blockers or aspirin, or both, were given on discharge. Of 267 patients, 158 (59%) were treated suboptimally in that they did not receive a secondary prophylactic drug to which they had no contraindication. For most patients this entailed underuse of one drug, but 17 (6%) of patients received no treatment. beta Blockers were 2.5 times less likely to be used than aspirin. Treatment was not associated with the age or sex of the patient, risk of further infarction, or hospital of discharge. Secondary prophylaxis after myocardial infarction is practised haphazardly. It should be offered to all patients who can tolerate it, after a trial period to assess any side effects of the drugs if necessary.