To prospectively study the prevalence of Lyme arthritis, 100 consecutive patients referred to a rheumatology out-patient clinic and 115 patients with a classified rheumatological disease were included. Individuals seropositive for antibody against the Borrelia burgdorferi sensu lato complex were examined for clinical signs or a history of Lyme borreliosis. Positive titres against B. burgdorferi s.l. were found in 7/100 and 15/115, respectively. Among the 100 referred patients. Lyme arthritis was diagnosed in five cases. Carpal tunnel syndrome was the presenting clinical feature in two of them. One of the 115 individuals with a previously classified rheumatological disease was re-classified as Lyme arthritis. All cases of Lyme arthritis improved after oral antibiotic treatment. This study revealed Lyme arthritis to be a common disorder in this part of Sweden and the diagnosis should be considered in patients with acute or recurrent episodes of mono- or oligoarthritis.