In a patient with chronic lymphocytic leukemia, multiple renal tubular defects developed in association with urinary excretion of K light chain proteins and peritubular deposits and casts of amyloid. Proximal tubular dysfunction, resembling adult Fanconi's syndrome, was suggested by an increased urinary loss of phosphate and urate and glycosuria. Renal tubular acidosis with hypokalemia and a marked impairment of the urinary concentrating mechanism were also observed.