Although Helicobacter cinaedi is the most commonly reported enterohepatic helicobacter in humans, there are no reports of virulence factors, and little is known about how it infects and causes disease. In this study, H. cinaedi isolates from humans and animals were examined for production of cytolethal distending toxin (Cdt). Cdt causes distention in cells and arrest in the G2/M phase of cell division. It is encoded by three genes: cdtA, cdtB, and cdtC. cdtB is the most conserved. All isolates except the American Type Culture Collection strain (identified as H. fennelliae) demonstrated Cdt activity and had a cdtB polymerase chain-reaction product homologous with known cdtB. Deduced amino acid sequences of cdtB showed a high (93%-99%) degree of similarity among the H. cinaedi isolates. H. cinaedi shares the production of Cdt with other enteric pathogens, including enterohepatic Helicobacter species. This is the first report of a putative virulence factor in H. cinaedi.