The aim of this study was to investigate IgG and IgM antibodies to ubiquitin in relation to Yersinia enterocolitica infection status in patients with AS. Twenty-eight AS patients (M:F 24:4, mean age 43.9 yrs, range 22–70 yrs, mean disease duration 15.9 yrs) and 35 healthy controls (M:F 31:4, mean age 52.1 yrs, range 22–80 yrs) were included. The levels of antibodies to ubiquitin and Yersinia O:3 and O:9 antigens were measured using specific ELISA. The results were expressed as optical density (OD) ratio. Antibody levels were assumed increased when the OD ratio was higher than mean OD ratio + 3SD in the control group. IgM antibodies to ubiquitin were found in five patients and one control (P < 0.05, Fisher's exact test). Anti-ubiquitin antibodies of IgG class were found in two cases, one AS patient and one control (NS). IgG antibodies to Yersinia serotypes O:3 and O:9 were present in eight and five AS patients, respectively (P < 0.001 and P < 0.05 vs. controls, Fisher's exact test). No IgM antibodies to Yersinia were found. High levels of IgG antibodies to Y. enterocolitica serotype O:3 were found in three out of five patients with high levels of IgM antibodies to ubiquitin, compared with five out of 23 patients with low levels of anti-ubiquitin antibodies (P=0.1231, NS). Antibodies to Yersinia serotype O:9 were found in three out of five patients with IgM antibodies to ubiquitin, compared to two out of 23 patients with low serum levels of IgM antibodies to ubiquitin (P < 0.05). The results suggest that Y. enterocolitica infection may induce antibodies to ubiquitin in a subset of patients with AS. This may be explained by the involvement of a newly discovered ubiquitin-dependent mechanism related to Y. enterocolitica virulence.