Acute anterior uveitis (AAU) is a disease of unknown etiology, although it is frequently associated with various autoimmune diseases. It has recently been shown in patients with ankylosing spondylitis (AS), a disease often associated with AAU, that antiserum raised against a particular isolate of Klebsiella pneumoniae would cross-react with lymphocytes possessing HLA-B27. The present study was performed to evaluate the response of lymphocytes of patients to K. pneumoniae and to determine the correlation with HLA-B27. The circulating immune complex levels in the serum of the patients were also determined for correlation with the presence of HLA-B27 antigen. We were able to demonstrate this cross-reactivity in the samples of AAU patients; however, we did find an increased immune complex level that was independent of HLA-B27 antigen. From our results, we conclude that although AAU is clinically associated with AS, the role of K. pneumoniae in these disease states remains unclear.