AbstractThis study reports on the caries susceptibility of tooth surfaces in 4294 adolescents (mean age 12.5 yr) during a 3‐yr, double‐blind clinical caries trial, conducted in Lanarkshire, Scotland, between 1988 and 1992. Children were selected on the grounds of dental maturity and past caries experience. Clinical examinations with mirror, CPITN probe and fibre optic trans‐illumination were carried out on the permanent dentition, with the buccal pits of mandibular molars and palatal pits of maxillary molars being recorded as separate sites. At baseline 6061 surfaces were decayed (1.0% of 601 160 surfaces examined), 20 160 (3.4%) filled, and 10 909 (1.8%) missing due to caries. The number of surfaces recorded as sound at baseline in subjects completing the study was 454 663. Of these 8176 (1.8%) new surfaces were decayed, 14 832 (3.3%) filled and 4000 (0.9%) missing at the final examination. Molar occlusal surfaces showed greatest susceptibility to attack, 35.8% of those at risk becoming carious in the course of the study. All buccal and lingual smooth surfaces showed a low susceptibility, but 8.8% of buccal and palatal pits developed caries. At the final clinical ‐ only examination, pit and fissure caries accounted for 48%, inter proximal surfaces for 39%, and smooth surfaces for 13% of caries prevalence. However, overall the contribution of these surfaces to 3‐yr increments was, 40%, 47%, and 13%, res