The purpose of this study was to investigate the etiology and characteristics of the ectopic eruption of first permanent molars. The subjects were 35 cases (27 maxilla and 8 mandible) with ectopically erupted permanent first molars, which were recognized in 24 out-patients (10 boys and 14 girls) visiting Tohoku University, Dental Hospital. The materials used for the analysis were dental casts, X-ray lateral cephalograms and panoramic radiographs, and both materials of the ectopic and normal groups were compared. The results were as follows: 1) There were no significant differences statistically between the mesio-distal crown diameters of the maxillary permanent first molar, central and lateral incisor, and all deciduous teeth in ectopic group and their mean values in Ono's and Otsubo's studies. 2) There were no significant differences as compared to all parts of the size of the dental arch in ectopic sides with those in the normal sides of same individuals at IIA and IIC of Hellman's developmental stage in dentition. 3) When the sizes of the dental arch in the ectopic group were compared with those of the normal group, the former were smaller at the distance between the mid-point and the canine mesial surface (Fig. 1-4), the ridge of the posterior region of the basal arch and most distal point of the primary second molar (Fig. 1-6) at the Hellman's dental age IIA, while the canine distance to the midline (Fig. 1-1), the dental arch perimeter from mid-point to the distal surface of primary second molar (Fig. 1-4+5), and the distance between the canine and primary second molar, div. X (Fig. 1-7) at Hellman's dental age of IIC were significantly smaller than the latter. 4) Regarding the comparison of the value of the changes in dental arch size between the ectopic group and the normal group from Hellman's dental age of IIA to IIC, the distance between the canine and primary second molar, div. X (Fig. 1-7) decreased markedly and the distance between the ridge of the posterior region of the basal arch and the most distal point of the primary second molar (Fig. 1-6) increased. 5) Results of the analysis of X-ray lateral cephalograms showed that the angle of eruption of first molars in the ectopic group was significantly smaller compared with normal group. 6) According to the cluster analysis on the calcification of tooth germ using panoramic radiographs, similarity among all of the samples was lacking in the ectopic group, especially congenital missing of the second molars and second premolars in the ectopic side and the delayed calcification of the ectopically erupted teeth were found in maxilla.