The cost effectiveness of early treatment with lisinopril in acute myocardial infarction (MI) was estimated using survival and cost data gathered prospectively during the hospitalisation of the overall population of patients enrolled in the third study of the Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto (GISSI-3), which assessed the efficacy of early (within 24 hours) treatment with an angiotensin-converting enzyme (ACE) inhibitor (lisinopril) for 6 weeks in a group of 19,394 relatively unselected patients with acute MI. A statistically significant reduction in 6-week mortality was achieved among patients treated with lisinopril when compared with patients allocated to the control group (absolute reduction in mortality: 7.5 +/- 3.6 lives saved per 1000 treated patients). The comparative cost-effectiveness ratio for the use of lisinopril, expressed as cost per additional survivor among patients randomised to receive lisinopril, was $US2080 per life saved (1993 values). The sensitivity analysis conducted to examine the effects of varying the estimated absolute reduction in mortality throughout its 95% confidence interval, which ranged from 14.6 to 0.4 lives saved per 1000 treated patients, showed that the cost-effectiveness ratios consequently vary from $US1121 to $US40,910 per life saved. The cost effectiveness of early treatment with lisinopril of a relatively unselected population of patients with acute MI compares very favourably with that of other therapies judged to be worthwhile by the medical community.