We describe the distribution of dental caries (DF), by age-group, between right and left homologous mesial and distal surfaces of permanent maxillary incisors. In any age-group above 15 yr, and for any pair of homologous surfaces, the proportion of the population with an affected (DF) right surface, approximately equals the proportion with an affected left surface. The distribution of attacks between each pair of homologous surfaces was studied in individual mouths, by age-group. For each pair of surfaces, the ratio of the numbers of people with asymmetrical attacks, to those with symmetrical attacks, falls asymptotically to a value that remains almost constant during adult life. These data, in common with those obtained from an earlier study of the distribution of caries between adjacent surfaces, imply that caries-resistant sites commonly occur on maxillary incisor teeth. The results are interpreted in terms of a new aetiological theory of dental caries.