ic as Audrey Heimler, * James New Hyde Park, N.Y. ** Ernest Lieber,” and Saul Kamen,** DEPARTMENT OF PEDIATRICS, SCHOOL OF MEDICINE; DEPARTMENT OF ORAL BIOLOGY AND PATHOLOGY AND DEPARTMENT OF DENTAL HEALTH, SCHOOL OF DENTAL MEDICINE; HEALTH SCIENCES CENTER, STATE UNIVERSITY OF NEW YORK AT STONY BROOK. Two children of Austrian Ashkenazic Jewish background, related as second cousins, have a variant of opalescent dentin in their deciduous teeth. This has been classified by Witkop as Erandywine isolate hereditary opalescent dentin and by Shields as dentinogenesis imperfecta type Ill. One of the children aiso has dysmorphic facial features, seizures, and severe mental retardation. Her mother has dysmorphic facial features and mild mental retardation. The mothers of both children and several other family members have classic opalescent dentin (dentinogenesis imperfecta type II). Radiographs of the deciduous and permanent dentitions of one mother showed obliterated pulp chambers. Confirmation of obliterated pulp chambers in the deciduous teeth of the mother of a child with Brandywine isolate hereditary opalescent dentin makes it unlikely that classic opalescent dentin and Brandywine isolate hereditary opalescent dentin are separate genetic disorders. Evidence from this family supports the hypothesis that Brandywine isolate hereditary opalescent dentin is a variant of opalescent dentin. (ORAL SURG. ORAL MED. ORAL PATHOL. 59~608-615, 1985) entinogenesis imperfecta (hereditary opalescent dentin) was probably first recognized by W.C. Bar- rett in 1882. The first published report describing the disorder as an enamel defect was that by Talbot in 1893. Early reports referred to “crownless teeth.” In 1905 Capdepont described a family in which there was an “abnormal friability of tooth substance” in three generations. He believed that both the enamel and the dentin were affected. Fargin-Foyelle and Malassez, working in France in 1908, were the first *Division of Human Genetics, Departiment of Pediatrics, Schneid- er Children’s Hospitai of Long Island Jewish Medical Center. **Department of Dentistry, Long Island Jewish Medical Center. to realize that the principal characteristics were due to abnormal development of dentin rather than enamel. ’ Dentinogenesis imperfecta occurs as an autosomal dominant trait with variableexpressivity either in persons with osteogenesis imperfecta or as a separate clinical entity in persons whohave none of the features of osteogenesis imperfecta. In the latter case, there is a very high degree of penetrance of the gene. Most investigators agree that dentinogenesis imperfecta in patients with osteogenesis imperfecta and isolated dentinogenesis imperfecta are separate disorders. Although there is no specific biochemical evidence to support this impression, Takagi and