The distribution of malignant melanoma among the 4 major body sites (head, upper limb, lower limb and remainder (trunk) was investigated for 37 white populations. Although UV radiation is generally considered to be the major aetiological agent, it was found that approximately 75% of the tumours occurred on the relatively unexposed body sites. However, the sex differences in the incidence of melanoma at the various sites corresponded in direction and magnitude with the patterns of exposures of the sexes. The greatest difference between the sexes was the higher incidence on the female lower limb (the regular wearing of skirts results in a considerable exposure), and the next largest was the higher incidence on the male trunk (men can remove their shirts easily, but do not do so regularly). The results indicate that UV radiation is a major cause of malignant melanoma, but suggest that the mechanism of induction may be complex. Several hypotheses, as well as the types of additional evidence required, are discussed.
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