The concentration of urinary free light chains was measured in 61 patients with systemic lupus erythematosus (SLE) by single radial immunodiffusion. The concentration was significantly increased in these patients, and was associated with hypocomplementemia and a high level of antibody to DNA. It also correlated with the clinical disease activity of SLE. It is postulated that increased free light chain in the urine of SLE patients is derived mostly from increased synthesis of light chain rather than from increased breakdown of intact immunoglobulins or decrease of catabolism during renal insufficiency. The measurement of urinary free light chains may offer information for evaluating disease activity in SLE patients.