Regional odontodysplasia is a relatively rare condition of undertermined etiology, involving a small group of adjacent teeth in both deciduous and permanent dentitions and characterized by the defective formation of both enamel and dentin. A boy aged 3 years,1 month was diagnosed as having regional odontodysplasia with a lesion of the maxillary left molars. The affected primary second molar showed severe hypoplasia, and had a periapical abscess. The distal alveolar bone was noticeably absorpted vertically. The maxillary left primary second molar was extracted after the diagnosis. Histopathological findings showed wide interglobular dentin and pulp stone or denticles. The maxillary left first molar revealed a symptom of severe perception hypersensitiveness immediately after eruption. The affected first molar was extracted because the root was malformed. The beginning of calcification of the maxillary left second premolar was seen at age of 6 years,9 months. The tooth crown of the second premolar was malformed, while the tooth root formation seemed to be sufficient. The affected second premolar erupted at the age of 14. The maxillary left second molar erupted on the medial and paratal side, and showed a cross-bite with the lower first molar as the opposing tooth. Orthodontic treatment was performed. The affected second premolar was restored with a chrome-steel crown as temporising restoration. The distal alveolar bone of the second premolar still showed diffused radiolucency on the X-ray. The root of the second premolar formed regularly.