Cell renewal in human epidermis was estimated after intradermal injection of thymidine-H3 in 21 subjects by means of serial biopsies and radioautographs. Results indicated 5% of germinative cells take up thymidine-H3 in 40 minutes. By seven days the number of labeled nuclei more than doubled and most were in the prickle layer. The first cell appeared in the granular layer in 10 to 14 days. After that a steady decline in number of labeled nuclei was observed. Epidermal cell renewal generally required 13 to 18 days. Epidermal cell turnover for man is similar to that reported for mouse and rat. The most important results challenge the older concept that epidermal cells divide and move in an orderly sequence. It now seems that mitosis and migration are random events.