A lower deciduous incioer exhibiting dentinogenesis imperfecta (D.I) obtained from a 6-year-old boy with osteogenesis imperfecta (Shields' Type I) was examined by means of light microscopy (LM), scanning electron microscopy (SEM), and X-ray microanalysis (XMA). With LM, the dentin displayed a sparse and irregular tubular pattern near the dentino-enamel junction (DEJ) and only few or no tubular structures in the area corresponding to the circumpulpal dentin. Between these two areas, cleft-like structures were characteristically noted. Structural irregularities in the dentinal tubules were also shown with SEM observation. XMA demonstrated that the distribution of both Ca and P in the dentin of DI teeth was apparently lower than that in the normal deciduous incisor used as a control. Specifically, an area along EDJ at a distance of 25-35 microns, corresponding to the mantle dentin, revealed extremely low or no distribution of the both elements. From the present observation, it is suggested that the generic disorder mainly involved in the primary odontoblasts and consequently results in the disturbance of calcification, especially that mediated by the matrix vesicles, and shortening of the cell life. After the death of these cells, the cells originate in, from the undifferentiated pulp cells may participate in the deposition of another irregular dentin.