A retrospective study of patients hospitalized for treatment of cystic lesions associated with retained third molars during a 10-year period showed a total of 32 cystic lesions in 25 patients. Twelve cysts were or had been infected, and seven were detected by routine radiographic investigation. Male to female ratio was 1.8, and mean age, 44,7 years; range, 16-76 years. Three of four lesions were found in the mandible. A compromised general health situation was found in eight patients. Mean largest diameter of the lesions was 48 mm; range, 26-72 mm. No differences in patient age or size of the lesions were found between infected and non-infected cases. All cysts were treated surgically, with a complication rate of 19%. Infection and age did not influence complication rate. Crude incidence rate was 0.038 per year per 1000 at risk—that is, persons more than 16 years old with at least one retained third molar present.