Various cytokines and growth factors may be involved in IgA nephropathy. To clarify whether interleukin-6 was a prognostic factor for this disease, we investigated interleukin-6 positivity of renal biopsy specimens and its relationship with the prognosis. The subjects were 90 patients with IgA nephropathy (42 males and 48 females with a median age of 32.7 ± 13.8 years). Renal biopsy specimens were stained for interleukin-6 using an enzyme-antibody method. Fifty-two of 90 patients showed glomerular positivity for interleukin-6. Among the patients positive for interleukin-6, 24-hour urinary protein excretion and serum creatinine levels were significantly higher at the time of biopsy than in the patients without interleukin-6 positivity, while creatinine clearance was significantly lower. In the interleukin-6-positive patients without steroid therapy, serum creatinine increased significantly after 1 year (Δs-Cr; 1.04 ± 0.45 mg/dl) and creatinine clearance decreased significantly (ΔCcr; –11.7 ± 3.2 ml/min) compared to the interleukin-6-negative patients without steroid therapy. Steroid therapy improved 24-hour urinary protein excretion, serum creatinine, and creatinine clearance in the interleukin-6-positive patients, while these parameters worsened without steroid therapy. On the other hand, the IL-6-negative patients showed no differences of clinical parameters irrespective of the presence or absence of steroid therapy. In conclusion, glomerular interleukin-6 positivity may be a prognostic factor and an indicator of the need for steroid therapy in IgA nephropathy.