Previously the NIRS oximeter OxyPrem was calibrated by comparison to the INVOS in a blood-lipid phantom. The aim of the present study was to test this calibration clinically. During vasculur occlusions in 10 adults and after birth in 25 term infants the relationship was OxyPrem = 1.24 x INVOS - 23.6% and OxyPrem = 1.15 x INVOS - 16.2% on the adult arm and infant head, respectively. The precsion during steady state was 4.0% (CI 3.4% to 4.6%) and 3.4% (CI 2.9% to 3.9%) on the arm, and 6.7% (CI 5.9% to 7.6%) and 4.7% (CI 3.5% to 5.9%) on the infant head for OxyPrem and INVOS, respectively. We conclude that the calibration on the blood-lipid phantom was unsuccessful in achieving agreement in clinical measurements.