A seasonal increased hair loss is frequently recognized in clinical practice. However, such an aspect of chronobiology has rarely been documented. To evaluate the seasonal influence on the hair cycle, trichograms of 2857 subjects who complained of an increased hair loss were collected during two consecutive years. An increased frequency of telogen effluvium was observed between July and October. The seasonal effluvium appears to be more summer effect than autumnal in regions whose latitude is over the Tropic of Cancer. The influence of ultraviolet light is possible. It might correspond to an actinic effluvium manifesting itself in some individuals among the whole population.