Objectives: To design and pilot a novel classification system for the assessment of caries lesion progression in bitewing radiography and to report rater agreement of the system. Methods: A classification system with drawings and text was designed to assess caries lesion progression. Guidelines for Reporting Reliability and Agreement Studies were used to study and report rater agreement. Pairs of posterior bitewing radiographs (baseline and 1-year follow-up) with different status concerning caries lesion progression were selected from files from public dental health clinics. 10 raters, 5 general dental practitioners and 5 specialists in oral and maxillofacial radiology were asked to assess the radiographs with the aid of the classification system. Seven raters repeated their assessments. Rater agreement was expressed as percentage of agreement and kappa. Results: Kappa for the interrater agreement of 10 raters assessing progression was 0.61, indicating substantial agreement. Agreement was moderate for progression in the outer half of the dentine (kappa 0.55) and within enamel (kappa 0.44). Pairwise interrater agreement varied (range 69-92%; kappa 0.42-0.84). For about half of the pairs of raters, kappa was substantial ($0.61). Intrarater agreement assessing progression was substantial (kappa 0.66-0.82). Conclusions: We demonstrated the applicability of the proposed classification system on caries lesion progression with respect to rater agreement. This system can provide a common framework for clinical decision-making on caries interventional methods and patient visiting intervals. Scientifically, this system allows for a comparative analysis of different methods of prevention and treatment of caries as well as of different caries risk assessment methods. Dentomaxillofacial Radiology (2016Radiology ( ) 45, 20160039. doi: 10.1259 Cite this article as: Senneby A, Elfvin M, Stebring-Franzon C, Rohlin M. A novel classification system for assessment of approximal caries lesion progression in bitewing radiographs. Dentomaxillofac Radiol 2016; 45: 20160039.