The clinical use and toxicity of intravenous lidocaine were surveyed in 750 hospitalized medical patients who received the drug for the treatment of cardiac arrhythmias. The majority of these patients were older than 60 years and most had diagnoses of cardiovascular disease. Lidocaine therapy was started within 48 hours of admission in 77 per cent of cases. Adverse reactions to lidocaine were reported in 6.3 per cent of patients, but only one in four reactions was considered life-threatening. Central nervous system disturbances and cardiovascular complications were the most common untoward effects. All but two adverse reactions occurred within the first 5 days of treatment. Adverse reactions were more common in elderly individuals, in those who died, and in those with long hospitalizations. Diagnoses of acute myocardial infarction or congestive heart failure, and low body weight were also associated with a higher frequency of unwanted effects. Serious adverse reactions to intravenous lidocaine were relatively uncommon. Patients with serious underlying disease or with diminished hepatic clearance of lidocaine appear to be predisposed to adverse effects from this drug.