Introduction Impacted mandibular third molars can be asymptomatic or cause some pathologies, such as distal caries and odontogenic cysts and tumours. This study investigates the prevalence of distal caries of the mandibular second molar adjacent to the mesioangular or horizontally partially erupted mandibular third molar and its relationship regarding decayed, missing and filled teeth (DMFT) risk group, age, side (left-right) and sex. Methods The study included patients who had an appointment for mesioangular or horizontally positioned and partially erupted mandibular third molar surgery. Before the surgery, the DMFT score, age, sex and side were recorded. After extraction, the second molar was clinically examined for distal caries. The prevalence of distal caries and the correlation between the DMFT risk group, age, sex and side were determined. Results The study was conducted on 514 patients and involved 639 teeth. Men and patients in the moderate- or high-risk group are significantly prone to developing distal caries of the second molar. No statistically significant relationship exists regarding age group, side and position. Conclusion Sex and DMFT risk groups affect the prevalence of distal caries in second molars adjacent to the mesioangular or horizontally partially erupted third molars. The prophylactic extraction should be considered in men and patients in the moderate- or high-risk group.