Nine children with isolated cleft mitral valve, aged 1 day to 12 years, were studied. The electrocardiogram showed a normal QRS axis in 5 subjects. Cardiac catheterization was performed in 4 patients and demonstrated severe mitral insufficiency in 3 but failed to clearly demonstrate a cleft mitral valve or gooseneck deformity. The mitral cleft was confirmed at operation in 2 patients. Two-dimensional echocardiography demonstrated a cleft dividing the anterior mitral leaflet into 2 portions in each patient. The mitral anulus was normally positioned and the atrioventricular septum present. Atrial and ventricular septa were intact. Features similar to anatomic studies such as accessory chordae and thickening of the edges of the cleft with increasing age were also seen. Two-dimensional echocardiography is the only method available to reliably diagnose isolated cleft of the mitral valve.