Two rare types of familial periodontitis, a localized form usually diagnosed in late adolescence, and a more generalized form with a latter mean age of diagnosis, have been analyzed with respect to genetic models currently favored in the dental literature. These include autosomal recessive and X-linked dominant (partial penetrance) inheritance. Since there is variation in severity, extent, age of onset, altered sex ratio of affected individuals, and a low population prevalence, it is not surprising that genetic mechanisms heretofore have not been revealed. We have compared the likelihoods of 33 kindreds ascertained through affected probands under the above genetic models. Our findings include (1) several families in which both forms of early onset periodontitis co-occur, making it unlikely that the clinical varieties of the disease have unrelated genetic causes; (2) the autosomal recessive model is far more likely than the X-linked dominant model. The superiority of the recessive hypothesis arises from the fact that there are only a few instances of affected individuals having affected parents and because the skewed sex ratio is shown to be incompatible with X-linked inheritance. These conclusions are largely insensitive to the assumptions of the analysis. We conclude that the X-linked dominant hypothesis is inadequate, and while the autosomal recessive model is by no means proven, it is clearly favored.