We randomised 128 patients who required operations on mandibular third molars and who had radiological evidence of proximity of the third molar to the canal of the inferior alveolar nerve to one of two operations: extraction [n=102], and coronectomy [n=94]. Some roots were dislodged during intended coronectomy and were therefore removed, resulting in two subgroups (successful coronectomy n=58, and failed coronectomy n=36). The mean (S.D.) follow up was 25 (13) months. Nineteen nerves were damaged (19%) after extraction, none after successful coronectomy, and three (8%) after failed coronectomy (p=0.01). The incidence of dry socket infection was similar in the three groups (10/102, 10%, 7/58, 12%, and 4/36, 11%, respectively). No root required removal or reoperation. To our knowledge this is the first clinical trial of the efficacy of coronectomy in preserving the inferior alveolar nerve. The length of follow up was about 2 years, which for the assessment of delayed eruption of the root fragments is not sufficient as this process may continue for up to 10 years. However, it seems that coronectomy reduces the incidence of injury to the inferior alveolar nerve without increasing the risk of dry socket or infection.