There is a need for a rapid predictor of potential clinical severity to guide therapy in patients with an acute overdose of tricyclic antidepressant drugs. We performed a prospective study of 49 such patients to observe the associations among serum drug levels, maximal limb-lead QRS duration, and the incidence of seizures and ventricular arrhythmias. Patients were divided into two groups on the basis of maximal limb-lead QRS duration. Group A (13 patients) had a duration of less than 0.10 second, and Group B (36 patients) had a QRS duration of 0.10 second or longer. No seizures or ventricular arrhythmias occurred in Group A. In Group B there was a 34 per cent incidence of seizures and a 14 per cent incidence of ventricular arrhythmias. All patients survived. Serum drug levels failed to predict the risk of seizures or ventricular arrhythmias accurately. Seizures occurred at any QRS duration of 0.10 second or longer (P less than 0.05), but ventricular arrhythmias were seen only with a QRS duration of 0.16 second or longer (P less than 0.0005). We conclude that determination of the maximal limb-lead QRS duration predicts the risk of seizures and ventricular arrhythmias in acute overdose with tricyclic antidepressants. Serum drug levels are not of predictive value.