Childhood sun exposure causes nevi (and melanoma), but there is little evidence regarding the effectiveness of sun protection strategies on the number of nevi. We previously found that boys but not girls receiving a school-based sun protection program had fewer nevi on their backs than controls. Here, we investigated whether specific program components (encouraging children to stay indoors in the middle of the day during summer, to wear clothing while outdoors, and to use sunscreen) were associated with fewer nevi. An observational analysis was done on data from a sun protection trial in 1,623 children in Perth, Australia. The outcome was number of nevi on the back 6 years after baseline, when the children were 12 years old. Information on sun protection was obtained by questionnaires 4 and 6 years after baseline. The data were analyzed by mixed-effects linear regression. The time spent outdoors between 11 a.m. and 2 p.m. and the proportion of total time outdoors that was between these hours were positively associated with number of nevi. Ratios of mean counts for doubling the respective measures were 1.09 [95% confidence interval (95% CI), 1.05-1.12] and 1.10 (95% CI, 1.05-1.14). Children whose backs were covered < 70% of the time while outdoors had 1.53 (95% CI, 1.34-1.75) times more nevi than children whose backs were always covered. Using sunscreen on the back when it was uncovered was not associated with number of nevi (P = 0.59). Children who stayed indoors in the middle of the day and wore clothing while outdoors had fewer nevi.