Langerhans cells (LCs) are dendritic cells located in the epiderm is with antigen-presenting capacities. We performed a quantitative analysis of LC density in the anterior chest skin of 286 autopsy cases, including 31 cases treated with radiation therapy. Skin specimens were stained by immunoperoxidase technique (PAP method) with an anti-S-100 protein antiserum. S-100 positive LCs were counted for comparison between non-irradiated and irradiated cases. In this study we noted that, 1) The decline in density of the LCs was age-related and dendritic processes were more prominent in younger groups. 2) The cases irradiated within one month before autopsy showed a reduction in LC density compared with age-matched controls. 3) The cases irradiated more than one month before autopsy demonstrated no consistent or definite tendency. It is suggested that ionizing irradiation as well as ultraviolet light may deplete the LC density in an acute phase. The possibility that radiation therapy alters immunological surveillance in the human skin is discussed.