Abstract
A 69-year-old male was admitted to our hospital due to rapidly progressive glomerulonephritis. A peripheral blood smear showed a marked increase in large granular lymphocytes. Flow cytometry analysis of the blood showed a marked increase in CD3-negative and CD56-positive natural killer (NK) cells. A renal biopsy showed a characteristic pathological pattern that involved endocapillary proliferation, a predominance of mononuclear cells and mesangiolysis. Prednisolone was administered, and the patient's renal function subsequently improved concomitant with the amelioration of NK cell proliferation. In our case, there was evidence of a strong association between NK cell proliferation and glomerulonephritis.