Twenty-nine patients with apparent ventricular tachycardia (VT) of left bundle branch block (LBBB) morphology were evaluated. Tachycardia was associated with an organic basis in 24 of 29 patients: 7 had Mahaim fibers of the nodoventricular type, 7 had arrhythmogenic right ventricular dysplasia, 5 had coronary heart disease, 3 had biventricular cardiomyopathy, and 2 had associated congenital heart disease. In many patients the underlying cardiac disease was not readily apparent. In the patients with a Mahaim fiber, the electrocardiogram taken during sinus rhythm was frequently normal. A reentry tachycardia with anterograde conduction over the nodoventricular fiber could mimic VT as diagnosed by the usual criteria; nodoventricular fibers were, therefore, often unsuspected before electrophysiologic evaluation. In patients with arrhythmogenic right ventricular dysplasia, cineangiography demonstrated abnormalities of the right ventricle, but only minor or no abnormalities of the left ventricle. Clinical and electrocardiographic features were not distinctive. Of the 29 patients, 22 had serious symptoms accompanying the tachyarrhythmia or had required cardioversion. Patients were followed up for an average of 20 months: 4 patients died. Thus, VT exhibiting an LBBB morphology is not uncommon and is frequently associated with organic heart disease, serious symptoms, and significant mortality. Right ventricular angiography and electrophysiologic study may clarify the diagnosis in these patients.