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To study reproducibility of cerebral tissue oxygenation index (cTOI) values in neonates and infants in a clinical setting using the NIRO 300 oximeter (Hamamatsu Photonics, Hamamatsu City, Japan).Clinical, observational study.University hospital, pediatric intensive care unit.Twenty neonatal and pediatric intensive care patients (age 0–190 days; median 4.5 days).Reproducibility of cTOI was measured at the lateral forehead of the patients.Sensor exchange experiments were performed by removing the sensor and reapplying another sensor (sensor 1 vs. sensor 2) at the same position. Simultaneous measurements, comparing cTOI values from the right and left forehead, were performed using both sensors. Corresponding sensor exchange experiments were performed within 10 mins. All tests were done under stable, steady-state cardiorespiratory conditions. Data were compared using Bland-Altman bias analysis and paired, two-sided Student's t-test (p < .05). Sensor exchange experiments and simultaneous left-to-right forehead measurements revealed only small mean differences (<5%) and no significant differences between corresponding values (p = .953/.164). However, Bland-Altman bias analysis revealed poor agreement with large 95% limits of agreement in particular for sensor exchange experiments (−17.8% to 17.6%) and less for simultaneous left and right measurements (−14.4% to 10.4%).The present study shows that cTOI measurements using the NIRO 300 oximeter at the lateral forehead of neonates and infants are not well reproducible under clinical conditions. This raises the question whether generally valid normal values can be defined with the used approach and makes it difficult to determine a normal range of cerebral oxygenation.