Ultraviolet-induced unscheduled DNA synthesis (i.e. repair synthesis) in human epidermal cells was measured as a function of age. Normal mammary skin specimens were obtained at surgery from 36 female patients, ranging in age from 17 to 77 years. The enzymatically isolated epidermal cells were analyzed for two parameters: (1) the number and percentage of cells carrying out repair synthesis, and (2) the rate of ultraviolet-induced unscheduled thymidine incorporation in individual cells. The results show that the percentage of epidermal cells capable of DNA excision repair synthesis does not decrease significantly with age, but that the rate of unscheduled DNA synthesis in individual cells decreases to a highly significant degree with advancing age.