Anaemia may increase the risk of tissue hypoxia in preterm infants. The effect of transfusion on circulation was studied in 33 preterm infants with a mean (SD) gestational age of 29 (5) weeks (range 26-34), birth weight 1153 (390) g (range 520-1840), and postnatal age of 48 (21) days (range 19-100). Packed cell volume, blood viscosity (capillary viscometer), cardiac output, and cerebral blood flow velocities in the internal carotid artery, anterior cerebral artery, and coeliac trunk (Doppler ultrasound) were determined before and after transfusion of 10 ml/kg of packed red blood cells. Transfusion increased packed cell volume from a mean (SD) 0.27 (0.45) to 0.37 (0.48). Mean arterial blood pressure did not change while heart rate decreased significantly from 161 (14) l/min to 149 (12). Cardiac output decreased from 367 (93) ml/kg/min to 311 (74) due to decrease in stroke volume from 2.28 (0.57) ml/kg to 2.14 (0.46) and in heart rate. There was a significant increase in systemic red cell transport (cardiac output times packed cell volume) by 17%, systemic flow resistance (blood pressure to cardiac output ratio) by 23%, and blood viscosity by 33%. Vascular hindrance (flow resistance to blood viscosity ratio) did not change significantly, thereby suggesting that neither vasoconstriction nor vasodilation occurred with transfusion. After transfusion blood flow velocities decreased significantly in the anterior cerebral artery by 23%, in the internal carotid artery by 8%, and in the coeliac trunk by 12%. Red cell transport estimated as products of blood flow velocities times packed cell volume increased significantly by 25% in the internal carotid artery and by 21% in the coeliac trunk. These results indicate that red cell transfusion improved systemic oxygen transport as well as oxygen transport in the internal carotid artery and coeliac trunk.