OBJECTIVES: The objective was to assess the dependence of the caries status of the adjacent approximal surface on the incidence of approximal caries. METHODS: At baseline, the material consisted of a cohort of 536 Swedish children. The individuals were followed through annual bitewing radiographs from the age of 11-13 to 21-22 years. A radiographic scoring system was used to assess the caries status of the surfaces: scores 0 and 1 = sound surface to score 4 = caries in the outer half of the dentine. A model was used to calculate the dependence of the caries status of adjacent approximal tooth surfaces. The unit of analysis was a pair of adjacent approximal surfaces and in all, 12 pairs of posterior approximal surfaces were analysed. RESULTS: The individual caries rates of the 24 posterior approximal surfaces ranged from 1.3 to 8.3 new caries lesions per 100 tooth surface-years. The caries rate of an approximal tooth surface depended on the caries status of the adjacent surface: a sound surface next to a sound surface had a relatively small risk of developing caries, while the risk increased 1.6-32.3 times if the adjacent surface was in a caries state as judged radiographically. The distal surface of the first molar developed caries more often than the mesial surface of the second molar. CONCLUSIONS: The caries rate of an approximal tooth surface was 1.6-32.3 times higher if the adjacent surface was in a caries state compared to when the latter was sound.