The purpose of this study was to determine the incidence of inferior alveolar nerve (IAN) damage after surgical removal of lower third molars, to identify the causes, and to construct a predictive model to assess the risk of IAN injury. We performed a nonrandomized forward prospective study of 946 consecutive outpatients subjected to surgical extraction of 1117 lower molars in the University of Barcelona Oral Surgery Department. Preoperative, intraoperative, and postoperative data were gathered, and suspected causal factors of IAN damage were identified by using nonparametric tests, the Pearson chi-square test, and the Fisher exact test. Logistic regression predicted the risk of IAN injury. Although only 1.3% of the extractions caused temporary nerve damage, 25% of the lesions were permanent. All of the following significantly increased the risk of IAN damage (P < .05): age, the radiologic relationship between the apices and the mandibular canal, deflection of the root when approaching the mandibular canal, distal ostectomy, the distance of the apices of the third molar to the mandibular canal, ostectomy, crown sectioning, pain during root luxation, primary closure of the wound, prolonged operating time, bleeding, exposure of the nerve, and postoperative ecchymosis. The first 4 factors were included in a predictive logit model. Patient age, ostectomy of the bone distal to the third molar, the radiologic relationship between the roots of the third molar and the mandibular canal, and deflection of the mandibular canal increased the risk of IAN damage. Older patients were at a higher risk for suffering permanent injuries.