Two patients with left ventricular thrombosis diagnosed by echocardiography are presented. The first patient was a 6-week-old girl with supraventricular tachycardia. Cross-sectional echocardiography showed a rounded and mobile structure protruding from the left ventricular wall. The girl was in heart failure and had signs of peripheral embolization. After sinus rhythm had been restored the thrombus diminished gradually and the girl recovered. The second patient was a newborn boy with severe aortic stenosis and a large massive thrombus along the left side of the interventricular septum. The boy died after valvotomy, the post-mortem examination confirmed the diagnosis of a large thrombus. Left ventricular thrombosis is uncommon in neonates and infants but may appear secondary to abnormal hemodynamics.