Several studies have provided evidence supporting the inheritance of juvenile periodontitis (JP) in a Mendelian fashion, and both X-linked and autosomal modes of transmission have been proposed. Re-examination of the evidence for the X-linked dominant hypothesis reveals that two assumptions were crucial in favoring the X-linked rather than an autosomal hypothesis of JP transmission. The first assumption was that females are more likely than males to have JP by ratios of approximately 2.5:1. The second key observation was the reported lack of father to son transmission of the trait. However, the data that these assumptions were based on may be interpreted differently. Although the number of females reported to be affected by JP is greater than the number of affected males, this is probably a reflection of the greater number of females incorporated into these studies. When the proportions of affected males and females are examined, rather than total numbers of affected individuals, the proportion of affected males and females is similar. Additionally, the reported lack of father to son transmission for JP appears to be the result of incomplete family data. The female ascertainment bias inherent in many JP studies, compounded by the lack of male participation in such studies, appears to have resulted in the perception of a lack of male to male transmission of JP. This lack of observed father to son transmission of JP in incomplete data sets has been regarded as proof of no male to male transmission for the trait. Studies of more complete family data, however, do document father to son transmission of JP.(ABSTRACT TRUNCATED AT 250 WORDS)