Of 14,344 hospitalised medical patients monitored in a drug surveillance program, 319 (2.4 %) received propranolol during one or more admissions. Angina and cardiac arrhythmias were the most common indications for therapy. Adverse reactions were attributed to propranolol in 30 (9.4 %) of the 319 patients. 22 of these reactions clearly involved impairment of cardiac function (bradycardia, heart block, hypotension, congestive heart failure), and in 8 cases they were considered life-threatening. Half of the adverse reactions occurred in patients taking low doses (less than 40 mg per day). In 11 cases, the reaction developed within 24 hours of the first dose. A literature survery of 797 ambulatory patients receiving oral propranolol revealed the most common adverse effects to be gastro-intestinal disturbances (11.2 % of patients), impaired cardiac function (6.9 %), and peripheral arterial insufficiency (5.8 %). Disturbances in central nervous system (CNS) function (fatigue, dizziness, impaired sleep, alterations in mood and thought) were also common. The data for propranolol suggest that treatment with β-adrenergic receptor blocking drugs is not without considerable risk, particularly in hospitalised patients with severe organic heart disease.