In a randomized and double-blind study of 116 patients with chronic heart failure (NYHA classes II or III) the effectiveness of captopril + hydrochlorothiazide (HCT) (group 1) and of digoxin + HCT (group 2) were compared. Treatment was effected for a 12-month period with a combination of 50 mg captopril (twice 25 mg daily, oral) and HCT, or 0.2 mg digoxin (twice 0.1 mg daily, oral) and HCT. In a pretreatment phase over 3-4 weeks the patients of group 1 were given an average HCT dose of 37.7 mg daily, whereas those of group 2 received 34.9 mg per day. At the end of the 12-month treatment period the patients in the captopril/HCT group had improved significantly more--by the criteria of echocardiographic intracardiac diameters, exercise tolerance and NYHA class--than those in the digoxin/HCT group. Change by a mean of one NYHA class had occurred in 61 patients (51.8%) of group 1 and in 47 (40.7%) of group 2 (P les than 0.01). These findings suggest that treatment of patients with mild to moderately severe chronic heart failure in sinus rhythm best be initiated with an angiotensin-converting enzyme inhibitor together with a diuretic rather than a digitalis-diuretic combination.