In a randomized, double-blind, cross-over study with plasma drug assays, 16 patients (11 men, 5 women; mean age 48.56 +/- 3.61 years) presenting with hypertrophic obstructive cardiomyopathy confirmed by echocardiography, left ventriculography and left intraventricular gradient measurement were treated with verapamil 480 mg/day or propranolol 320 mg/day. Both treatments produced functional improvement (p less than 0.01) which was more distinct with verapamil (NS). No changes in cardiothoracic index, echocardiographic parameters and Sokolow's index were observed. Mean total heart work during exercise, which was 1,197.27 +/- 135.89 watts before treatment, increased to 1,260.91 +/- 146.60 watts under propranolol (NS) and to 1,344.09 +/- 171.06 watts under verapamil (NS). Maximum heart rate during exercise, which was 162.3 +/- 3.46 beats/min before treatment, was reduced to a greater extent by propranolol (122.1 +/- 6.6 beats/min; p less than 0.001) than by verapamil (147.7 less than 5.08 beats/min; p +/- 0.01). The two treatments did not significantly modify ventricular arrhythmia, arterial and capillary pulmonary pressures, mean aortic pressure and left ventricular end-systolic pressure. Cardiac index, unchanged under verapamil, fell from 2.98 +/- 0.16 1 X min-1 X m-2 to 2.60 +/- 0.11 1 X min-1 X m-2 under propranolol (p less than 0.05). The left intraventricular gradient present in 5 patients at rest and during exercise, was reduced by both drugs. The gradient under isoprenaline (n = 16), which was 162.07 +/- 18.77 mmHg before treatment, fell to 93.86 +/- 24.48 mmHg with propranolol (p less than 0.05) and to 128.86 +/- 18.22 mmHg with verapamil (p less than 0.05). Left ventricular ejection fraction, mean circumferential fibre shortening speed and compliance coefficient remained unchanged under both drugs (NS). Left ventricular diastolic function, evaluated by radioisotope angiography in the last 9 patients, was most often improved by verapamil (NS). Verapamil was better tolerated generally and by the heart than propranolol. No correlation was observed between plasma verapamil levels and clinical results. Low plasma propranolol levels were often noted in non-responders, suggesting a need for treatment with high doses. It is concluded that at the dosage level used in this study propranolol and verapamil were equally effective, but there were individual variations in best response to one or the other of these two drugs.