Objective To investigate the relationship between ambient ultraviolet radiation (UV) exposure and number of melanocytic nevi in children.Methods A longitudinal study of nevi was conducted in 1614 children in Perth, Australia. Children had nevi counted on the back, face and arms at ages 6, 10, and 12 years. Erythemally effective UV irradiance was used to estimate ambient exposure from their places of residence before entry to the study at age 6 years. Data on UV radiation were derived from satellite measurements of ozone and atmospheric reflectivity.Results At baseline, the response rate was 70%. At age 10, 90% of those recruited had nevus counts and at age 12, 69%. Children who had migrated to Perth from geographical locations with higher erythemal irradiance had, on average, 1.34 (95% confidence interval, 1.16-1.54) times as many nevi on the back at age 6, 1.29 (1.13-1.49) as many at age 10 and 1.10 (0.92-1.30) times as many at age 12. No significant associations were seen for the face or arms.Conclusion Ambient solar UV exposure in early childhood is positively associated with number of nevi and nevi develop soon after exposure to sunlight.