The light source of a modern operating microscope gives a clear operative field, but the possibility of thermal impairment by the strong light is substantial. We aimed to elucidate the potential risk of such strong light on transfer of free flaps and the optimal application of light. First, the thermal effect of clinically used microsurgical illumination was examined by direct irradiation of a thermometer. Secondly, the thermal influence on living tissue of microsurgical light was evaluated using a model of vascularised and non-vascularised pedicled groin flaps in rats. Thirdly, the influence of strong light on the transfer of free flaps was investigated using a model of a free groin flap in rats. Fourthly, histological alteration was investigated by a light microscopy and scanning electron microscopy. With direct irradiation of a thermometer, temperatures reached values in excess of 80 degrees C with maximal xenon light illumination from 20 cm. The mode of illumination significantly influenced the results of free flaps. Prolonged irradiation of the vascular pedicle in particular resulted in an increase in the number of failures, supposedly caused by venous occlusion. Bright illumination contributed to efficient microvascular procedures, but we confirmed its potential risk. Our findings may benefit all surgeons engaged in transfer of free flaps, as potential impairment caused by excessive operative irradiation is prevented simply by recognition of its harm.