The incidence of the coexistence of left ventricular false tendons and premature ventricular contractions (PVCs) was evaluated prospectively. Over 14 months, left ventricular false tendons were found in 71 (6.4%) of 1117 consecutive patients examined echocardiographically. Two types of false tendons were observed: longitudinal, from the ventricular septum to the posteroapical wall (n = 62), and transverse, between the septum and the lateral wall (n = 9). Among 62 patients with PVCs and no underlying heart disease, false tendons were detected in 35 (56%); 28 had unifocal and seven had bifocal PVCs. Episodes of ventricular tachycardia were documented in one of the 28 patients with unifocal PVCs and in one of the seven patients with bifocal PVCs. These PVCs were poorly controlled by antiarrhythmic drugs but easily suppressed by exercise. Left ventricular false tendons were detected in 36 patients on routine echocardiographic examinations performed in the other 1055 subjects, and 10 of these patients were judged to have no underlying heart disease. PVCs were detected in two (20%) of these 10 patients. Although a definite conclusion that left ventricular false tendons are arrhythmogenic cannot be derived from these results, the unexpectedly high incidence of the coexistence suggests that left ventricular false tendons may be an etiologic factor in the development of PVCs, especially the rate-dependent and medically uncontrollable PVCs seen in apparently healthy individuals.