Doppler colour flow mapping provides a non-invasive method for studying patterns of blood flow in the cardiovascular system. We undertook a prospective study using this technique in order to document normal and abnormal patterns in the human fetus. A group of 50 consecutive normal fetuses were examined with colour flow mapping. Patterns of flow that could be readily identified in the majority included inflow at the atrioventricular valves, outflow through the arterial valves and flow in the aortic arch and arterial duct. Flow across the oval foramen and in the pulmonary veins was identified in approximately half the patients. Colour Doppler also proved helpful in the detection and definition of cardiac anomalies. A series of 74 cardiac malformations, seen during 1988, were examined with colour flow mapping. The presence or absence of flow across valves could be confirmed, valvar incompetence demonstrated and the direction of blood flow displayed across both normal and abnormal communications. Colour Doppler studies in the human fetus can thus improve the understanding of the characteristics of flow of blood and add to the accuracy of antenatal diagnosis of congenital heart disease.