The effects of oral disopyramide 150 mg 4 times a day were compared with propranolol 40 mg 4 times a day and placebo in 10 patients with hypertrophic cardiomyopathy and resting obstruction (7 patients) or latent obstruction (3 patients), in a randomized double-blind crossover design; each drug was given for a period of 4 days. As determined from echocardiographic evaluation of systolic anterior motion of the mitral valve, the subaortic pressure gradient was decreased from 61 +/- 20 mm Hg with placebo to 5 +/- 15 mm Hg with disopyramide (p less than 0.01), and 30 +/- 30 mm Hg with propranolol (p less than 0.01). Disopyramide was more effective than propranolol (p less than 0.01). Disopyramide and propranolol both shortened left ventricular ejection time from 352 +/- 51 ms with placebo to 314 +/- 26 and 322 +/- 41 ms, respectively (p less than 0.01). Preejection period was lengthened from 93 +/- 35 ms with placebo to 119 +/- 25 ms with disopyramide, but was unchanged by propranolol at 98 +/- 23 ms. Disopyramide increased exercise duration versus placebo (10.4 +/- 2 vs 9.6 +/- 2 minutes, respectively (p less than 0.05), whereas propranolol produced no significant change (8.8 +/- 2 minutes).