Introduction Distal caries in lower second molars has been associated with mesioangular third molars. Caries detection and restoration can be difficult. If caries progresses, root canal treatment or extraction of the second molar can be necessary.Aims To identify the prevalence of caries in lower third molars and the distal aspect of corresponding lower second molars in patients referred for lower third molar assessment.Methods Analysis of OPG X-rays for 420 consecutive patients (776 third molars) referred to three maxillofacial centres over a five month period.Results Thirty-four percent of third molars were mesioangular. There was radiographic evidence of distal second molar caries in 42% of these. When unerupted mesioangular third molars were excluded this increased to 54%. There was no difference in age or dental health of these patients compared to the whole group. There was no angulation of the mesioangular third molar for which distal caries in the second molar was more likely.Conclusion Distal caries in lower second molars related to a mesioangular third molar is a common finding in oral and maxillofacial patients in secondary care, especially if the third molar is fully or partially erupted. If such a third molar is left in situ, close monitoring and regular bitewing radiographs are recommended.