OBJECTIVE: To compare the effects on renal function of captopril and enalapril in elderly patients with chronic heart failure. DESIGN: A multi-centre double-blind parallel-group comparison of the two angiotensin-converting enzyme (ACE) inhibitors, captopril (12.5 mg bid) and enalapril (2.5 mg bid). SUBJECTS: 80 elderly patients with chronic heart failure (41 in the captopril group, 39 in the enalapril group). MAIN OUTCOME MEASURES: The blood pressure and pulse rate response to the first dose of ACE inhibitor was assessed in all patients. Glomerular filtration rate (GFR) was measured radioisotopically by 99mTcDTPA or 51CrEDTA clearance after three and six months of each treatment. Subgroups were assessed for effective renal plasma flow (33 patients), exercise tolerance (25 patients) and by a symptom-oriented questionnaire (45 patients). RESULTS: No serious adverse effect on GFR was noticed. There was no significant difference between the two treatments in the mean baseline GFR or in changes from baseline at three and six months (captopril mean baseline GFR 49.6 ml min-1 1.76 m-2, enalapril 54.7 ml min-1 1.76 m-2; mean change (95% confidence interval) at three months captopril 12 ml min-1 (+3.0, +21.0), enalapril -2 ml min-1 (-13.0; +9.0); mean change at six months, captopril 3.7 ml min-1 (-6.7; +14.2), enalapril -6.0 ml min-1 (-21.0; +9.4). Significantly more patients given captopril had an improvement in GFR during the study period (26/31 compared with 20/31 enalapril-treated patients at three months, p = 0.0096, and 23/30 compared with 15/27 at six months, p = 0.021). There were no significant changes in effective renal plasma flow. Three patients treated with enalapril developed symptomatic hypotension within three days of starting treatment. Quality of life questionnaires revealed more gastrointestinal symptoms in the enalapril group (p = 0.039). CONCLUSIONS: Captopril seems marginally preferable to enalapril in the treatment of chronic heart failure in elderly patients.