OBJECTIVES--To assess whether the reduction in left ventricular dilatation after acute myocardial infarction obtained by early administration of angiotensin converting enzyme inhibitors depends on continuous treatment. DESIGN--Prospective observational and cross sectional study of withdrawal of randomised treatment with enalapril or placebo. PATIENTS--106 patients on 6 months trial treatment after an acute myocardial infarction. MAIN OUTCOME MEASURES--Left ventricular volumes and ejection fraction as assessed by echocardiography and circulating proatrial natriuretic factor (1-98) before and 4-6 weeks after withdrawal of treatment. RESULTS--There were no significant changes (mean (SD)) in left ventricular systolic (0.7 (4.7) ml/m2) and diastolic (0.4 (6.6) ml/m2) volume indices, ejection fraction (-0.9 (6)%), and proatrial natriuretic factor (172 (992) pmol/l) after withdrawal of enalapril. The significantly lower left ventricular volumes observed with 6 months of enalapril therapy after acute myocardial infarction, as compared with placebo, were maintained 6 weeks after drug withdrawal. CONCLUSION--The results show that the benefit of 6 months of enalapril treatment initiated early after myocardial infarction is maintained for at least 6 weeks after drug withdrawal, suggesting that the treatment effect on left ventricular structure is not reversed by changes in loading conditions caused by subsequent drug withdrawal.