The aim of this study was to evaluate the prevalence of second molar distal caries in a Turkish population and to determine the factors that affect it. Clinical records and panoramic radiographs of partially erupted mandibular third molars were reviewed in this retrospective study. The analysis outcome measures were the patients' age, second molar distal caries, third molar angulation and second and third molar contact point localization. Prevalence of second molar distal caries in the population was 20%. This prevalence was 47% when the third molar had an angulation of 31-70 degrees (majority of mesioangular third molars) and 43% at 70-90 degrees (all horizontal third molars). The contact point on the second molar amelocemental junction and increasing age had significant effects on caries formation. The results revealed that second molar distal caries justifies prophylactic third molar removal and partially erupted third molars that have an angulation of 30-90 degrees with a contact point on the amelocemental junction should be removed to prevent second molar distal caries.