In an earlier study, we found that T-cell lines (TCL) from five patients with treatment-resistant Lyme arthritis preferentially recognized Borrelia burgdorferi outer surface protein A (OspA), but TCL from four patients with treatment-responsive arthritis only rarely recognized this protein. Dominant T-cell recognition of an arthritogenic OspA epitope is one way in which the immune response against OspA might be involved in the pathogenesis of treatment-resistant Lyme arthritis. In an effort to test this hypothesis, we mapped the epitopes of 31 OspA-specific TCL and five T-cell clones derived from the synovial fluid or peripheral blood samples of three patients with treatment-resistant Lyme arthritis. Although each patient's TCL recognized a broad array of OspA peptides with different individual patterns, two regions of OspA were dominantly recognized. Each patient's TCL dominantly recognized a C-terminal epitope of OspA, ranging from amino acids (aa) 214 to 233 in one patient to 244 to 263 in another, and the TCL of all three patients dominantly recognized an epitope between aa 84 and 113. These dominant regions were confirmed by clonal analysis in one patient. Thus, the region of OspA between aa 84 and 113 was the dominant T-cell epitope shared by these three patients with treatment-resistant Lyme arthritis. If the T-cell response to OspA is involved in the pathogenesis of treatment-resistant Lyme arthritis, and epitope contained within aa 84 to 113 is a potentially arthritogenic epitope.