In a prospective study 1,106 impacted mandibular third molars were removed from 687 patients. Clinical, radiographic, and surgical factors were recorded. Postoperatively, we examined the modalities of common sensation in order to assess sensory deficit. The patients were followed up, until complete restitution occurred, or, if the sensibility failed to recover, for at least 6 months. A total of 3.6% of the operated sides demonstrated impairment of labial sensation, and 2.1% of lingual sensation. The vast majority of these disturbances subsided completely during the follow-up period. The incidence of persisting sensory diminution after 6 months was 0.91% for the inferior alveolar, and 0.37% for the lingual nerve. However, the degree of the persisting deficit was slight in most instances. The relationship between the recorded factors and the alteration of sensation was analysed by using the chi2 test. For the inferior alveolar nerve we found the patient's age, the development of the roots, the degree of impaction, and the radiographic position of the nerve canal to be significantly correlated to sensory deficit, as well as the surgical procedures in the depth of the socket and the intraoperative opening of the mandibular canal. As far as the lingual nerve is concerned, general anaesthesia and the individual operator were the main factors predictive of nerve damage.