Impaction status of a mandibular third molar (M3) leads to some pathological conditions. Objective: This retrospective study was conducted to compare the effect of erupted (e-M3) and impacted mandibular third molars (i-M3) on caries in the adjacent second molar and external root resorption (ERR). Methods: We used cone beam computed tomography (CBCT) images of 200 patients with a mean age of 28.19 +/- 8.3 years who had an e-M3 on one side of the mandible and an i-M3 on the other side of the mandible. All the images were evaluated for the existence of caries and ERR. Results: Caries was detected in 32.5% of cases of i-M3 and in 21% of cases of e-M3 (p = 0.01). ERR was present in 18.5% of cases of i-M3 and no resorption was observed in cases of e-M3 (p = 0.00). Conclusions: Caries and ERR in cases of i-M3 can be reliably identified via CBCT scan. Among patients with caries in the i-M3 group, the degree of caries was significantly lower on the other side of the mandible. Instead of prophylactic removal of e-M3, periodic clinical and radiologic examinations are advised.