The distribution and transmission of Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans in 4 families were studied. The families were included, based on the isolation of P. gingivalis from a young child or adolescent. The probands of these 4 families were: a 5-year old periodontally healthy boy; a 17-year old girl with severe generalized juvenile periodontitis; an 11-year old girl with prepubertal periodontitis; 2 sisters, 5 and 17-years old, with untreated severe periodontitis as a component of the Papillon-Lefèvre syndrome. All members of the 4 families were examined clinically and microbiologically for the presence of P. gingivalis and A. actinomycetemcomitans. Most of the parents appeared to be adult periodontitis patients; the parents of one proband were edentulous. Results showed that in all cases at least one of the parents was positive for P. gingivalis. On the basis of indistinguishable restriction endonuclease patterns (REPs) of P. gingivalis and A. actinomycetemcomitans isolates from parents and their children, and distinct REPs from unrelated individuals, the present study indicates that P. gingivalis and A. actinomycetemcomitans were transmitted between parents and their children.