OBJECTIVE : To determine whether functional near-infrared spectroscopy (fNIRS) allows monitoring fatigue in radiologists during prolonged image interpretation. METHODS : Nine radiologists participated as subjects in the present study and continuously interpreted medical images and generated reports for cases for more than 4 h under real clinical work conditions. We measured changes in oxygenated hemoglobin concentrations [oxy-Hb] in the prefrontal cortex using 16-channel fNIRS (OEG16ME, Spectratech) every hour during the Stroop task to evaluate fatigue of radiologists and recorded fatigue scale (FS) as a behavior data. RESULTS : Two subjects showed a subjective feeling of fatigue and an apparent decrease in brain activity after 4 h, so the experiment was completed in 4 h. The remaining seven subjects continued the experiment up to 5 h. FS decreased with time, and a significant reduction was observed between before and the end of image interpretation. Seven out of nine subjects showed a minimum [oxy-Hb] change at the end of prolonged image interpretation. The mean change of [oxy-Hb] at the end of all nine subjects was significantly less than the maximum during image interpretation. CONCLUSIONS : fNIRS using the change of [oxy-Hb] may be useful for monitoring fatigue in radiologists during image interpretation.