A 30-year-old male who presented with acute renal failure was found to have acquired immunodeficiency syndrome (AIDS). Although sonography and computerized tomography did not show urinary tract dilatation, obstructive renal failure was demonstrated by retrograde pyelography. Relief of obstruction(s) due to encasement of the renal pelves and ureters with histiocytic lymphoma led to immediate return of normal renal function. Although the etiology of renal failure in this patient is highly unusual, the high incidence of lymphoma in patients with AIDS should make tumor-related renal disease a consideration in all such patients with renal dysfunction.