Aim An increasing number of children undergo magnetic resonance imaging requiring anesthesia or sedation to ensure their immobility; however, magnetic resonance imaging may increase body temperature whereas sedation or anesthesia may decrease it. We investigated changes in body temperature in children who underwent sedation or anesthesia for magnetic resonance imaging. Methods Children aged 12 weeks-12 years undergoing anesthesia and magnetic resonance imaging were included in this prospective observational study. Tympanic body temperature was measured before and after magnetic resonance imaging, and the difference between measurements was calculated. Associations between the temperature difference and patient- or procedure-related factors were evaluated with linear and logistic regression analysis. Results A total of 74 children were included, of whom 5 (7%) had a temperature increase >= 0.5 degrees C. Mean temperature difference was -0.24 degrees C (SD 0.48) for the entire group and -0.28 degrees C for the youngest children (0-2 years). The temperature difference correlated positively with the duration of imaging (unadjusted coefficient 0.26, 95% confidence interval (CI), (0.01; 0.52)). Conclusion In this study of sedated or anesthetized children undergoing magnetic resonance imaging, clinically relevant increases in body temperature above 0.5 degrees C were only found in a few patients. However, longer imaging duration tended to be associated with increased body temperature.