The 1920s and 1930s represented an extraordinary time in the shaping of modern attitudes towards ultraviolet light. Dermatologists and other physicians today are still confronting the effects of changes in social behavior that occurred at this time. The discovery that ultraviolet wavelengths played a role in vitamin D synthesis in the skin ushered in a period of enormous popularity for ultraviolet light exposure. A variety of other medical claims were soon made for ultraviolet radiation, including that it increased resistance to disease. The field of phototherapy rapidly expanded, and its use was employed by proponents for a host of unlikely medical conditions. Exposure to sunlight or ultraviolet lamps was widely promoted as a form of preventive medicine. Home sunlamps gained popularity and were aggressively marketed to the public. A suntan, which had previously achieved limited popularity, now was viewed as de rigueur in the United States and Europe. The role that medical advocacy of ultraviolet light exposure played in initially advancing the practice of sunbathing is not commonly appreciated today. Ironically, public health recommendations of the time were often diametrically opposed to those being made at present, since sunlight exposure is currently recognized as the major preventable cause of cancer of the skin.