Using the IL-6 dependent hybridoma, MH60.BSF2, we measured urinary levels of interleukin 6 (IL-6) in 29 patients with active lupus nephritis. We detected IL-6 activity in the urine of 24 (83%) of 29 patients before the initiation of therapy. The median value of urinary IL-6 levels in patients with a histologic diagnosis of WHO class IV on renal biopsy was significantly higher than that in patients with other classes (p < 0.01). After treatment, urinary levels of IL-6 decreased significantly (p < 0.001). These data suggest that urinary levels of IL-6 may be a valuable tool for monitoring the progression of lupus nephritis.