Our clinical and serological studies have shown that Lyme arthritis is not a rare manifestation of Lyme borreliosis in Europe. The significance of serological findings for Lyme arthritis have more often remained uncertain in consideration of potential differential diagnoses than was to be expected on the basis of specificity controls. Various joint manifestations in the course of Lyme borreliosis have been distinguished. The succession or the coexistence of intermittent attacks of arthralgias and arthritis has been pointed out as particular indications of Lyme arthritis. We have noted diffuse hand and finger swelling as a striking feature of early Lyme arthritis. Generally, Lyme arthritis has been attributable to Stage 3 of the disease. The most pathognomonic manifestation has been intermittent knee arthritis. The pattern of joint involvement has shown similarities to that in postenteric and postveneral reactive arthritides, in particular as regards the occurence of dactylitis (sausage digits) and heel involvement. However, we have not seen sacroiliitis and Reiter's syndrome. As distinguished from typical Lyme arthritis, peculiar joint manifestations have been noted in association with acrodermatitis chronica atrophicans. Typing of Class I and II histocompatibility antigens did not give evidence of any immunogenetic basis for Lyme arthritis.