The role of Campylobacter jejuni cytolethal distending toxin (CDT) on clinical outcome after gastroenteritis was investigated. Clinical data, blood serum samples, and Campylobacter spp. isolated, from each of 30 patients were collected over a period of 6 months. The CDT encoding genes, cdtABC, characterized by PCR, revealed that all but one of the C. jejuni strains had the wild-type sequence. Sequencing of cdtABC from this strain showed two major deletions. From all of the strains, CDT titers were determined, and toxin neutralizing antibodies were documented using an in vitro assay. Three of the thirty clinical isolates, including the one with the mutant cdtABC coding genes, did not have a detectable CDT activity. Analyzing the relationship between CDT titer, serum neutralization of CDT, and the clinical outcome showed that campylobacteriosis caused by CDT-negative strains was clinically indistinguishable from that of patients infected with an isolate that produced high levels of CDT. These results suggest that CDT does not solely determine severity of infection and clinical outcome.