The purpose of this study was to determine the prevalence of Actinobacillus actinomycetemcomitans (Aa) in individuals from families where at least one individual has an early onset form of periodontitis. Twenty-three families with 73 subjects were evaluated in this study. Forty-seven early onset periodontitis subjects outside the 23 families were also studied. Prevalence of detection of Aa in family members ranged from 49% to 66% among groups constituted by clinical findings indicative of no loss of attachment, adult periodontitis, generalized severe juvenile periodontitis, or localized juvenile periodontitis. Whether the data were analyzed by subject or by site, no statistical differences could be found in prevalence (proportion of positive samples) among those clinical groups. The only significant difference was that localized juvenile periodontitis subjects had higher concentrations of Aa in their Aa-positive sites than did the other clinical groups. The prevalence of Aa-positive sites in subjects without attachment loss, but who are members of families in which early onset periodontitis is represented, was much higher than in other reports where periodontally healthy subjects were not related to early onset periodontitis cases. This suggests that Aa may be transmitted among members of families in which one or more members has an early onset form of periodontitis.