Two patients developed acute renal failure with Leptospira pomona infection. In both patients the oliguria persisted for two weeks. Renal biopsies showed a marked focal mononuclear infiltrate. One patient appears to have made an uninterrupted recovery. The other patient has developed interstitial fibrosis and impairment of renal function. In one patient an abnormal band composed of IgM appeared in the γ-globulin area. This persisted for three weeks.