Porphyromonas gingivalis has been associated with severe forms of periodontitis. The question can be raised about the origin of this suspected periodontal pathogen. The purpose of the present investigation was to study the possibility of transmission of P. gingivalis between spouses. 18 patients were selected with severe periodontitis and colonized with P. gingivalis. 10 of their spouses appeared to be colonized with P. gingivalis too. 8 of the patients and their spouses were investigated further clinically and microbiologically. Microbiological evaluation revealed mostly high %s of P. gingivalis in the pockets, not only of the patients (5-48% of the cultivable flora), but also in 7 of the 8 spouses (0.2-61%). Furthermore, this species was isolated often from the saliva, the tongue, the buccal mucosa and the tonsillar area from both patients and spouses. For typing purposes, bacterial DNA was isolated, treated with the restriction endonucleases Bam HI or Pst I, after which the DNA fragments were separated by agarose gelelectrophoresis. With one exception, each individual was colonized with only one clonal type of P. gingivalis. The DNA patterns of all P. gingivalis isolates from unrelated individuals were found to be distinct. In contrast, in 6 of the 8 couples, the DNA patterns of isolates from husband and wife were indistinguishable. From these data, it can be concluded that it is most likely, that P. gingivalis can be transmitted between spouses. It remains to be investigated whether transmission of P. gingivalis is a risk factor for developing periodontal destruction.