An analysis of factors influencing the initiation of carious lesions on specific tooth surfaces over a 4-year period in children between the ages of 11-12 years and 15-16 years is presented. Approximately 1000 children, resident in the County of South Glamorgan, Wales, were assessed for caries status and oral cleanliness in 1980 when aged 11-12 years and again in 1984 when aged 15-16 years. On both occasions, the children completed detailed questionnaires on dental health-related topics. Surfaces which were sound when the children were 11-12 years were identified and subsequently awarded a score of zero if they remained sound at 15-16 years or a score of one if they had developed carious lesions or had been filled. For each child, a mean mouth caries initiation score was computed for specific groups of surfaces, namely pit and fissure surfaces in posterior teeth, approximal surfaces in posterior teeth, buccal and lingual smooth surfaces of all teeth and approximal surfaces of anterior teeth. One-way analysis of variance and multiple regression techniques revealed that a number of factors had a significant influence on the initiation of caries. The factors and their level of significance varied between the surfaces. However, relatively little (less than 6 per cent) of the total variance in caries initiation score could be explained by the identified factors. Overall, more lesions developed in pit and fissure surfaces in posterior teeth than in the other surfaces included in the analyses.