A premature infant with nonbacterial endocardial thrombosis and embolization of the cerebral circulation is presented. The patient was born at 32 weeks with a birthweight of 1480 g. Pregnancy was complicated by toxemia and placental infarctions. Echocardiographic examination by 12 hours after birth revealed a large mass in the right atrium compatible with endocardial thrombus. Follow-up echocardiograms showed a small mass in the left atrium and a decrease of the right atrial mass by day 7. Echoencephologram studies were normal on day 1. An echodense area in the subcortical white matter was seen by day 2. Other echodensities were detected by day 10. Computerized tomography studies showed areas of decreased attenuation in both hemispheres. Lung perfusion scan and renal-liver ultrasound studies were normal. All these cerebral and cardiac lesions were asymptomatic. Real-time ultrasonography is useful to diagnose asymptomatic endocardial thrombosis and its cerebral complications in premature infants.