Factor analysis and multiple regression analysis were made to establish a method for the prediction of the unerupted third molar size. The samples were dental casts of 138 adults (69 women and 69 men) who had all their teeth from the central incisor to the third molar, at least in one side of each maxillary and mandibular dentition. Mesiodistal and buccolingual tooth crown sizes were measured with sliding calipers. Five factors were selected from the factor analysis on the tooth crown sizes. Multiple regression analysis with the third molar as the dependent variable was carried out. Independent variables were selected on the basis of the factors obtained from the factor analysis. Two kinds of multiple regression equations were obtained (the first molar was used as an independent variable in one equation and the second molar was used in the other). The accuracy of the prediction was highest when the lateral incisor, the second premolar, and the second molar were used as independent variables in the maxillary dentition, and the central incisor, the first premolar, and the second molar in the mandibular dentition. The mean of the absolute values of the differences between the predicted values and the actual values was 0.5 mm in the maxillary third molar, and 0.42 mm in the mandibular third molar.