Dermatologists for over half a century have believed that ultraviolet rays in sunlight are the chief carcinogenetic factor in cutaneous cancer and in aging of the skin. Clinical findings alone are inadequate to support this belief. However, supporting evidence from fields such as astronomy, meteorology, ethnology, etc., plus the converging evidence from experimental laboratory work,1 now offer support for the observations of the earlier dermatologists. Early Empirical Reports Now Supported As early as 1894, P. G. Unna2 ascribed to prolonged exposure to sunlight a dermatosis of the exposed skin of sailors which he called "Seemanshaut" or "sailor's skin." A few years later, Hyde,3 in 1906, and Dubreuilh,4 in 1907, published the first extensive evidence in support of the theory that sunlight might be the cause of skin cancer. Dubreuilh4 observed that women workers in Bordeaux vineyards developed cancers on unprotected parts of their faces, while