It has been postulated that site-specific variation in melanocytic nevus density and size is explained by differential response to sunlight. We observed the density and size of nevi at different body sites in relation to age, phenotype, latitude, and other measures of ultraviolet exposure. A standard protocol was used to assess nevi, phenotype, and sun exposure in 1123 Australian schoolchildren at 3 contrasting latitudes. Associations with phenotype (red hair, skin reflectance, sun sensitivity, and tanning) varied by body site. In Queensland, gender differences in nevus density on the back and lower limbs, unrelated to sun exposure, were similar to gender differences for melanoma. Small nevi (2-4 mm) were most dense on the arms, whereas large nevi (≥5 mm) were most dense on the posterior trunk where they were related to age, decreasing latitude, male sex, and freckling. Our findings support the hypothesis of site-specific differences in nevus proliferative potential. (J Am Acad Dermatol 2003;48:367-75.)