Twenty-nine patients with ankylosing spondylitis (AS) were studied in an attempt to evaluate the role of T lymphocytes in this disease and a possible influence of treatment. The proportions of various T cell subpopulations in blood were assessed with monoclonal antibodies. Before treatment the proportions of Leu-4+ cells and Leu 3a+ cells were decreased while Leu-2a+ lymphocytes appeared in normal proportions. Leu-7+ cells appeared in increased proportions. An increased proportion of Leu-M1 positive cells were identified in the lymphocyte preparation from the patients, possibly reflecting the presence of activated granulocytes. Activation of the different cell types was studied with double staining technique. No activated Leu-3a+ or Leu-2a+ lymphocytes were present in blood when the patients were analyzed as one group, but when divided into subgroups according to inflammatory activity, the highest levels of activated Leu-2a+ lymphocytes were found in the group with active disease. Functional assays measuring DNA synthesis of T and B cells were normal. After three months treatment with sulphasalazine the patients showed clinical and laboratory improvement. The proportion of activated Leu-2a+ cells decreased during treatment, but no other changes occurred in the lymphocyte markers or lymphocyte functional tests. A patient control group showed no clinical improvement nor any changes in T cell markers. Our results support the concept that AS is a disease which affects the lymphocyte system and that the improvement induced by sulphasalazine may involve actions on this system.