During a retrospective review of 70 consecutive intensive care unit admissions for tricyclic antidepressant (TCA) overdose, we identified 12 patients (17%) who had simultaneously ingested a neuroleptic agent. We then compared several clinical and laboratory features of patients who had ingested TCAs alone with those patients who had also taken a neuroleptic drug. The 12 patients in the TCA-neuroleptic group, when compared with patients in the TCA-only group, showed a markedly higher prevalence of first-degree atrioventricular block (p less than 0.001), a significantly higher prevalence of prolongation of the QRS duration (greater than 0.10 sec) (p less than 0.05), and a threefold increase in the prevalence of QTc prolongation (p less than 0.05). Our data indicate that coingestion of neuroleptics and TCAs, when compared with the ingestion of TCA alone, may significantly increase the risk of adverse cardiac consequences. We discuss the possible mechanisms for these electrocardiographic changes as well as their potential implications.