In 1986 and 1987 nine patients with a raised antibody titre to Yersinia pseudotuberculosis were found. Two of these results were almost certainly due to the cross reactivity between some Y pseudotuberculosis serogroups and some salmonella serotypes. Of the other seven patients with otherwise unexplained Y pseudotuberculosis serology, three suffered from severe arthritis and two had symptoms suggestive of sacroiliitis. Our data and published reports suggest that reactive arthritis might follow both Y enterocolitica and Y pseudotuberculosis infections.