OBJECTIVES AND BACKGROUND. Severe primary tricuspid regurgitation in the adult is a rare finding. This study describes the diagnostic findings and the treatment of an isolated congenital cleft of the anterior leaflet of the tricuspid valve as the morphologic substrate for severe tricuspid regurgitation. METHODS. The clinical, echocardiographic findings and the follow-up findings of five patients (all male, 20 to 56 years old) with this disorder are described. Four of the five patients underwent cardiac surgery that confirmed the diagnosis. RESULTS. In three of five patients, exertional fatigue was the limiting symptom (New York Heart Association functional classes II and III). The clinical findings included a holosystolic murmur and supraventricular arrhythmias in all patients. Cardiac catheterization, performed in four patients, yielded the incorrect diagnosis of Ebstein's anomaly in three. In one patient the cleft was associated with an atrial septal defect of the secundum type. In four of five patients successful reconstruction of the tricuspid valve with a DeVega annuloplasty was performed. One patient had a partial excision of the right atrium, and one had a closure of a coexisting atrial septal defect. One patient refused operation. CONCLUSIONS. Tricuspid valve anomalies can be accurately identified by Doppler echocardiography. Surgical repair is the treatment of choice in patients with severe tricuspid regurgitation due to a congenital cleft of the anterior leaflet of the tricuspid valve.