Campylobacter concisus is an emerging pathogen of the human gastrointestinal tract. Recently, a significantly higher prevalence of C. concisus DNA and higher levels of antibodies specific to C. concisus was detected in children with Crohn's disease when compared with controls. The aim of this study was to identify C. concisus immunoreactive antigens. Proteins from C. concisus were separated using two-dimensional gel electrophoresis, and sera from 10 C. concisus-positive children with Crohn's disease were employed for immunoprobing. The patients' sera reacted with 69 spots, which corresponded to 31 proteins identified by mass spectrometry. The proteins were functionally classified as involved in chemotaxis, signal transduction, flagellar motility, surface binding and membrane protein assembly. Although the individual patients' sera reacted to different sets of proteins, common antigens that were recognized by all patients were flagellin B, ATP synthase F1 alpha subunit, and outer membrane protein 18. Cross-reactivity between proteins of the Campylobacter genus was tested using patients' sera absorbed with Campylobacter showae, Campylobacter jejuni and Campylobacter ureolyticus. Most of the C. concisus immunoreactive proteins identified in this study showed cross-reactivity with other species except for three antigens. In conclusion, this study has identified C. concisus proteins that are immunoreactive within patients with Crohn's disease.