Twenty-one patients with muscular subaortic stenosis were treated with oral propranolol for periods of 6 to 34 months for a total of 42.5 patient years. The average follow-up was 2 years. Four patients with latent obstruction became asymptomatic on propranolol therapy. Of the 17 patients with resting obstruction, 7 improved, 2 were unchanged, 5 deteriorated, and 2 died during the period of treatment. The 7 patients with resting obstruction who are still improved on propranolol have had relatively short periods of treatment (average 15 months), and none experienced the degree of improvement that occurred in the patients with latent obstruction. This study indicates that propranolol is most effective in patients with muscular subaortic stenosis who have latent obstruction. It is of limited value in patients with resting obstruction because the benefit of propranolol therapy in the majority of these patients is eventually overtaken by progression in the disease.