A cytolethal distending toxin (CDT) found in Actinobacillus actinomycetemcomitans inhibits the eukaryotic cell cycle, which may contribute to the pathogenic potential of the bacterium. The presence of the cdtABC genes and CDT activity were examined in 40 clinical isolates of A. actinomycetemcomitans from Brazil, Kenya, Japan and Sweden. Thirty-nine of 40 cell lysates caused distension of Chinese hamster ovary cells. At least one of the cdt genes was detected in all strains examined. The three cdt genes were detected, by PCR, in 34 DNA samples. DNA from one strain from Kenya did not yield amplicons of the cdtA and cdtB genes and did not express toxic activity. Restriction analysis was performed on every amplicon obtained. PCR-RFLP patterns revealed that the three cdt genes were conserved. These data provided evidence that the cdt genes are found and expressed in the majority of the A. actinomycetemcomitans isolates. Although a quantitative difference in cytotoxicity was observed, indicating variation in expression of CDT among strains, no clear relationship between CDT activity and periodontal status was found.