Objectives: The purpose of this study is to investigate the features and histological structures of the human fused double upper lateral incisors and to clarify the forming process of it. Methods: The materials used in this study were a fused double upper left lateral incisors of a Japanese female patient aged twelve. It was observed by binocular microscopy and roentgenography. Horizontal ground sections were prepared and observed by polarizing microscopy, phase microscopy, fluorescent microscopy, microradiography, and scanning electron microscopy. Tangential re-ground surface of the enamel was observed by scanning electron microscopy. Results: Two upper lateral incisors were mesiodistally stood side by side. These were fused at the roots and cervical parts of the crowns. The macroscopic features of the distal upper lateral incisor were more degenerative than those of the mesial one. The angle and curve symbols, mesial and distal margins of the labial surface, and mesial and distal marginal ridges of the lingual surface of the mesial upper left lateral incisor revealed them of the opposite (right) upper lateral incisor. No discontinuous histological structure was observed at the junction of both teeth. A fluorescent line observed on the ground section of the dentins in two lateral incisors was jointed at the junction of both teeth. The distance, between the dentoenamel junction and the fluorescent line in the labial dentin, at the distal part of the mesial upper lateral incisor was longer than that at the mesial part of it. In comparison with the mesial upper lateral incisor, the distal one showed a high degree of distortion in the enamel rod sections. Conclusion: These results suggest that this fused tooth was not formed by a fused two tooth germs but by an imperfectly divided tooth germ.

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