Study of a large five-generation kindred from southern Maryland revealed that type III dentinogenesis imperfecta (DGI-III) and a localized form of juvenile periodontitis (JP) were both segregating as autosomal-dominant traits. Linkage analyses demonstrated that these were two distinct clinical entities, making this family the first documented instance of an autosomal-dominant form of JP. Since the locus for the more common form of dentinogenesis imperfecta (DGI-II) is on chromosome 4q [Ball et al, 1982], a linkage analysis of genetic and chromosomal markers on chromosome 4 was undertaken. The results suggested that the locus for the DGI-III subtype is located a similar distance from the Gc locus (theta = 0.12) as the distance previously observed between Gc and DGI-II loci (theta = 0.11) [Ball et al, 1982; Conneally et al, 1984]. Most likely the two DGI subtypes are determined by genes at closely linked loci, by allelic genes, or by the same gene with the variable expression in different families. In addition, close linkage between the Gc locus and that determining the autosomal-dominant form of JP was observed in this family (theta = 0.05). The known map of chromosome 4q and our analysis of the markers tested suggested the gene order to be 4cen----JP----Gc----DGI----MNS----qter with a large distance (at least 15 cM) between 4cen and JP.