The purpose of this research was to study the prevalence of ectopic eruption of the first permanent molars and possible etiologic factors. A group of 4,232 Thai students, from 6 to 9 years old, was examined. The prevalence of ectopic eruption of the first permanent molars in the subjects was 0.75%. Both the severity of the ectopic eruption and the amount of root resorption on the second primary molars were more pronounced in the maxilla than in the mandible. The important etiologic factors were the eruption path of the first permanent molars relative to reference lines and the size of the mandibular second primary molars. The amount of proximal caries did not seem to affect the prevalence of ectopic eruption.