Cases in which stones are found on both sides at the same time are alone considered. They form 9·4% of the admissions for stone in the upper urinary tract at St. Peter's Hospital. Figures from Continental clinics vary from 11% to 14%, while those obtained from post-mortem records are nearly 50%. Four groups of cases are considered: (1) Calculi due to a special diathesis, e.g., cystine stones. (2) Aseptic bilateral calculi. (3) Infected bilateral calculi. (4) Cases complicated by anuria. (1) Cystinuria should be treated medically, by diet and alkalis, but stones may form, in spite of treatment. They can be passed easily, and operation is only indicated when impaction occurs. (2) The calculi are comparatively small, and it is rare to find more than one on each side. If the renal function is approximately the same on both sides, simultaneous removal is advisable. When this is impossible, the interval between the two operations should not exceed fourteen days. When the function is unequal, the first operation should be performed on the more damaged kidney. (3) When both sides are infected, the calculi are often very large, and the kidneys severely damaged. Infection is usually the primary factor, but its source cannot always be determined. The symptoms are slight, often merely persisting pyuria. If the value of both kidneys is the same: (a) no operation may be possible; (b) pelvic stones should be removed; (c) “stag horn” calculi are best left alone, unless there is evidence of fluid distension of the kidneys. When the function is unequal, an absolutely useless pyonephrotic kidney should be removed or drained, but if urine is secreted by both it is advisable to operate on the better kidney first. (4) In cases of calculous anuria, the obstruction is usually found in the upper portion of the ureters. An attempt should be made to relieve it by passing ureteric catheters. If it fails, or if the anuria recurs, immediate operation is necessary. The kidney last obstructed should be drained. The stones should be removed as soon as the effects of the anuria have passed.