Chronic Lyme arthritis that is unresponsive to antibiotic therapy is associated with an increased frequency of the HLA-DR4 specificity. To determine whether the immune response to a particular polypeptide of Borrelia burgdorferi may be associated with treatment-resistant chronic Lyme arthritis, we correlated the clinical courses and HLA-DR specificities of 128 patients with Lyme disease with their antibody responses to spirochetal polypeptides. Antibody reactivity was determined by Western blotting (immunoblotting) with sonicated whole B. burgdorferi and recombinant forms of its outer surface proteins, OspA and OspB, as the antigen preparations. Of 15 patients monitored for 4 to 12 years, 11 (73%) developed strong immunoglobulin G responses to both OspA and OspB near the beginning of prolonged episodes of arthritis, from 5 months to 7 years after disease onset. When single serum samples from 80 patients with Lyme arthritis, were tested, 57 (71%) showed antibody reactivity to recombinant Osp proteins; in contrast, none of 43 patients who had erythema migrans or Lyme meningitis (P < 0.00001) and 1 of 5 patients who had chronic neuroborreliosis but who never had arthritis (P = 0.03) showed antibody reactivity to these proteins. Among the 60 antibiotic-treated patients with Lyme arthritis, those with the HLA-DR4 specificity and Osp reactivity had arthritis for a significantly longer time after treatment than those who lacked Osp reactivity (median duration, 9.5 versus 4 months; P = 0.009); a similar trend was found for the HLA-DR2 specificity. For other HLA-DR specificities, arthritis resolved within a median duration of 2 months in both Osp-reactive and nonreactive patients. We conclude that the combination of the HLA-DR4 specificity and OspA or OspB reactivity is associated with chronic arthritis and the lack of a response to antibiotic therapy.