Excessive exposure to sunlight is known to damage the skin. However, the emphasis of most studies has been on the consequences of sunlight exposure to fair-skinned individuals, and the situation of people with heavy skin pigmentation residing in, or migrating to, geographic locations with limited sunlight incidence has been largely neglected. Recent epidemiological studies suggested the hypothesis that sunlight deprivation, and the associated reduction in the circulating levels of vitamin D3 (vit D3) derivatives may lead to the increased incidence of the carcinomas of the breast, colon, and prostate. Two endocrine pathways may mediate these effects. The pineal function can potentially be involved, but the formation of vit D3 derivatives is gaining credibility as a mechanism for the retardation of cancer progression. Evidence is accumulating that such compounds, e.g., 1,25-dihydroxyvitamin D3 (1,25D3) induce differentiation of several neoplastic cell types, arrest or retard their proliferation, and act as chemopreventive agents in animal carcinogenesis. We also propose that the antineoplastic effects of vit D3 derivatives are exerted at several steps in tumor progression and that immunomodulating effects of 1,25D3 may contribute to these effects of sunlight. The recent findings that common cancers, e.g., carcinoma of the prostate and the breast, behave more aggressively in black Americans than in white Americans may be explained on this basis. Although more data are needed on the effects of sunlight on the circulating levels of 1,25D3, a corollary of this hypothesis is that there should be no broad condemnation of moderate sunlight exposure, as it may be available in insufficient amounts to some Americans.