The material consisted of 527 randomly selected children and adolescents from the community of Jönköping, Sweden, who in 1983 reached the age of 3, 5, 10, 15, or 20 years. The degree of incisal or occlusal tooth wear was evaluated for each single tooth in accordance with the following criteria: score 0 = no wear or negligible wear of enamel; score 1 = obvious wear of enamel or wear through the enamel to the dentin in single spots; score 2 = wear of the dentin up to one-third of the crown height; and score 3 = wear of the dentin more than one-third of the crown height. In the age groups 3 and 5 years the primary dentition was studied, and in the age groups 10, 15, and 20 years the permanent dentition. In the 3-year-old children 63% and in the 5-year-olds 19% had no or slight incisal or occlusal wear in the primary dentition. In the permanent dentition the corresponding figures for the 10-, 15-, and 20-year-olds were 78%, 51%, and 35%, respectively. The 5-year-olds had the highest percentage of primary teeth with incisal or occlusal wear related to existing teeth in accordance with criteria 1-3 (32.2%), and the 10-year-olds had the lowest score for permanent teeth (2.5%). There were small or no differences in tooth wear between the sexes in these age groups. Eighteen children (17%) among the 5-year-olds had one or more teeth with wear scored 2 in the primary dentition, and one individual had 4 primary teeth scored 3. The corresponding figures for the 3-, 10-, 15-, and 20 year-olds were 2%, 1%, 7% and 6%, respectively. No permanent teeth with wear scored 3 were found in these age groups. The number of teeth with incisal or occlusal wear increased with age both in the primary and in the permanent dentition.