Pre-existing left ventricular dysfunction in patients undergoing induction therapy with cyclophosphamide prior to bone marrow transplantation (BMT) has resulted in overt heart failure in a large number of patients. This fact excludes the majority of such patients from consideration for BMT at many centers. We sought to determine if prophylactic treatment with the angiotensin converting enzyme inhibitor enalapril prevents this deterioration in pre-existing left ventricular dysfunction. We treated six consecutive patients with initial left ventricular ejection fractions (LVEF) by radionuclide gated blood pool imaging (RGBP) of < 50% (42 +/- 7%) with enalapril 5 mg orally twice per day started 48 h prior to induction therapy and continued throughout the follow-up period. Serial RGBP imaging demonstrated an increase in LVEF in all patients to 54 +/- 6% (P < 0.005). No patient experienced clinical deterioration during a follow-up period of 18 +/- 11 months. We conclude that prophylactic treatment with enalapril may prevent deterioration in pre-existing mild left ventricular dysfunction during BMT.