Background: Surface temperatures of fibreoptic phototherapy pads using a high intensity blue light-emitting diode (LED) light source have not been studied. Objectives: The aim of this study was to measure the temperature of LED fibreoptic phototherapy pads during phototherapy in a bench-top study, and to determine temperature effects on babies during phototherapy. Methods: A commercially available LED fibreoptic phototherapy system was tested. In a bench-top setting, pad surface temperatures were measured before, during and after a 12-h period of phototherapy (10 different LED light box-pad combinations). A prospective, cohort study of well babies at >34 weeks' gestation receiving phototherapy was then conducted to determine changes in pad and body temperatures during a 90-min phototherapy period. Results: In the bench-top study, the mean (95% CI) pad temperature was 21.8°C (21.5-22.1) before lights, 27.0°C (26.5-27.5) after 12 h of lights, and 22.1°C (21.9-22.4) 8 h after turning off the lights (F = 366.1, p < 0.0005). The magnitude of change in pad temperature with phototherapy was linearly correlated with irradiance (r = 0.89, p < 0.0005). The pad plastic covering absorbed 13% of blue light from fibres. In the clinical study, the warmest pad temperature during phototherapy was 38.9°C. Axillary temperature increased by a mean (95% CI) of 0.3°C (0.1-0.5), p < 0.019, and exceeded 37.5°C in 4 babies. Conclusions: LED fibreoptic phototherapy pads are heated by high-intensity blue light. The thermal environment and temperature of babies should be monitored closely during LED fibreoptic phototherapy. A temperature probe placed between the skin and the pad will not accurately reflect the core temperature during fibreoptic phototherapy.