We investigated interleukin 6 (IL-6) mRNA expression in peripheral blood T-cells obtained from 36 patients with IgA nephropathy (IgAN), 36 patients with other glomerulonephritides and 24 healthy age-matched controls. The majority of patients with IgAN had increased IL-6 mRNA expression by their T cells; no IL-6 mRNA was detected in T cells obtained from patients with other glomerulonephritides or normal controls. A positive correlation was noted between the IL-6 mRNA level and quantity of protein excretion in the urine, histopathological findings, and renal function. However, there was no significant correlation between IL-6 mRNA expression and the IgA-immune complex titer, serum IgA level or blood pressure. mRNA levels in T cells obtained from patients with grade III or IV renal histopathological findings were significantly higher than in those with grade I or II histopathology. In addition, mRNA levels in T cells obtained from patients with more than 1.0 g/day proteinuria were markedly higher than those with less than 1.0 g/day proteinuria. We also studied the clinical course of 11 patients with IgAN during hospitalization. The IL-6 mRNA levels in these patients decreased gradually, as did proteinuria, after treatment. These studies suggest that abnormally regulated IL-6 mRNA expression in peripheral blood T cells may be associated with disease activity in IgAN.