By the use of radioimmunoassay, urinary β2-microglobulin concentration was mesured in 49 patients with systemic lupus erythematosus (SLE). The urinary β2-microglobulin was significantly elevated in SLE (p<0.01). The level of urinary β2-microglobulin did not correlate with the level of serum complement, antibody to DNA or the massive proteinuria. It was associated with urinary free light chain and the degree of renal tubular damage, but not with the glomerular lesions observed in renal biopsy specimens. The elevated level of β2-microglobulin in urine was most likely due to increased excretion of β2-microglobulin from involved tubulus or direct secretion from lymphocytes infiltrating the interstitium. The measurement of the level of urinary β2-microglobulin is useful for estimating the extent of renal interstitial damage in patients with SLE.