The proinflammatory cytokine interferon gamma (IFG) is elevated in body fluids of multiple sclerosis patients but its variation range is broad. The reason for this wide scatter of IFG production is not yet known. We looked for the relation between clinical parameters such as disability, exacerbation frequency, disease duration, course of the disease and IFG producing blood lymphocytes. Forty-one consecutive, clinically stable multiple sclerosis patients with primary relapsing course of the disease and without immunomodulatory or immunosuppressive treatment in the last 3 months were investigated for IFG in blood lymphocytes by flow cytometry. A significant positive correlation between IFG production and disability (r = 0.45, P50.01, Spearman's rho coefficient) was found. Pathophysiological implications and therapeutical relevance of this unexpected finding are discussed.