The objective was to assess the ability of near infrared spectrophotometry (NIRS) to detect changes in tissue oxygenation due to alterations in oxygen delivery. Ten hemodynamically stable preterm neonates with a median gestational age of 27.9 weeks (range 25.1-31.2), a median birth weight of 840g (range 690-1310), and a postnatal age of 29 days (range 2-45) were included in this prospective trial. Tissue oxygenation of the lower leg was measured by NIRS and the oxygen extraction index (OEI) was calculated prior and after a transfusion of 10-20 ml/kg body weight packed red blood cells. The OEI decreased from 0.31 (range 0.13-0.39) to 0.24 (range 0.12-0.36, p<0.005). This decrease correlated positively with the weight matched amount of packed red cell transfusion (r2=0.40, p<0.05) and with the increase in hematocrit (r2=0.58, p<0.005). The OEI obtained by a NIRS may allow to monitor changes in tissue oxygenation.