To test the hypothesis that children living in subtropical and tropical environments have more naevi than those of similar ethnicity living in temperature countries, a comparative study of melanocytic naevi in 111 schoolchildren from Brisbane, Australia, and 222 from Glasgow, Scotland, was carried out. All children were aged 13-15 years, of European ancestry, and had spent most of their lives at latitudes of less than 30 degrees S (Australia) or greater than 30 degrees N (Scotland). Using an identical protocol, all naevi of 2 mm or more in diameter occurring on the right arm were counted by either a highly experienced research nurse in Brisbane, or a dermatologist in Glasgow. Hair and eye colour, and facial freckling, were assessed by the examiner, and axillary skin colour of children in both cities was measured using the same reflectance spectrophotometer. Children in Brisbane had significantly more naevi than those in Glasgow (P < 0.05), after adjusting for complexion variables. The difference in the geometric mean number of naevi on the arm was much greater among boys (7.7 vs. 4.4, in Brisbane and Glasgow, respectively) than among girls (7.3 vs. 6.7). This has parallels with the sex differences in melanoma at later ages in the two countries. Besides country of residence, freckles and innate skin colour were the most significant predictors of large numbers of naevi, whereas red hair had a significant protective effect. Overall, these data on prevalence of naevi in children from contrasting environments provide some evidence in support of the theory that naevus development is related to the level of sun exposure in childhood and adolescence.