Systemic lupus erythematosus (SLE), a progressive autoimmune disorder, is associated with chronic stimulation of various components of the immune system. Since cell-mediated immunity is also activated, we were interested to test for abnormalities in tryptophan metabolism in SLE which may result from activation of indoleamine 2,3-dioxygenase by cytokines released during the immune response. We measured serum tryptophan and kynurenine concentrations in 52 patients with SLE as well as serum neopterin as an indicator for the degree of immune activation. Compared to controls, we found significantly decreased tryptophan and increased kynurenine concentrations in SLE. The extent of tryptophan catabolism correlates with neopterin concentrations or with the disease activity index. Tryptophan depletion may be associated with neurologic/psychiatric disturbances in patients suffering from SLE.