The number of melanocytic nevi is the most important independent risk factor for cutaneous melanoma. Aim of our study was to add information to the controversial discussion on the role of chronic-moderate and intermittent-high sun exposure and sunburns for the development of melanocytic nevi by the use of a large longitudinal study. A longitudinal study with a 3-year follow-up was conducted in 1,232 young children 2–7 years of age attending 78 public nursery schools in Bochum and Stuttgart, Germany. Total body nevus counts, assessment of pigmentary features and nevus counts on arms of parents were carried out. Parents underwent a standardized interview concerning sun exposure, sunburns and sun-protective behavior. Applying multiple linear regression analysis higher numbers of incident nevi were associated with host factors like light skin complexion (skin Type II vs. IV, p = 0.022) and freckling of the face (p < 0.001), with parental factors like nevus counts on mothers' (p < 0.001) and fathers' (p = 0.004) arms and at least one parent being of German descent (p = 0.006), and with environmental factors like intermittent-high sun exposure during holidays (p < 0.001) and chronic-moderate ultraviolet radiation at home (p = 0.007). Sunburns were a significant risk factor for nevus development (p = 0.005). Total cumulative sun exposure seems to be the crucial environmental risk factor for the development of nevi, whether the child is exposed to chronic-moderate or intermittent-high ultraviolet light doses. Public health education should focus primarily on avoiding sun exposure especially in children with fair skin and parents with high nevus counts. © 2004 Wiley-Liss, Inc.