Renal involvement in Hansen's disease was evaluated in 94 Portuguese patients, average age and duration of disease of 47.6 and 6.8 years respectively. Sixty-seven were studied retrospectively and 27 prospectively; renal biopsy was obtained in 4, fat-tissue needle aspiration for amyloidosis in 20, and tubular function was tested in ten. Mild proteinuria and/or haematuria was found in 33 patients, the severity increasing during erythema nodosum leprosum reactions, but without overt nephritic or nephrotic syndrome. Two patients had renal amyloidosis on biopsy and two more were confirmed by fat biopsy, a 10.5% incidence in those studied prospectively; all but one were of the lepromatous type, with frequent bouts of erythema nodosum leprosum. The two other renal biopsies showed mesangial glomerulonephritis, and one unexplained acute tubular necrosis; none had immune deposits by immunofluorescence. Proximal acidification was always normal, distal acidification tested by bicarbonate infusion was abnormal in one of nine patients, and six of nine patients had concentration defects. Leprosy causes frequent urinary sediment changes and concentration defects, usually without clinical expression; proteinuria and/or glomerular involvement is mainly due to amyloidosis.