Aprindine hydrochloride is an antiarrhythmic drug now under investigation. The drug is most effective in prophylaxis of ventricular dysrhythmias. Previously recognized toxic effects include neurological side effects, nausea, vomiting, as well as a reported association with agranulocytosis. To our knowledge, this is the first report of agranulocytosis associated with documented elevated serum concentrations of aprindine. Further correlation of drug levels with toxic reactions may help in establishing the mechanism of bone marrow suppression.