To discuss the subject of the differential diagnosis of ovarian cysts from other pelvic and abdominal tumors would require a consideration of all the pathological growths that may occur in the abdomen. About every form of abdominal tumor has been mistaken for ovarian cyst. Differential diagnosis is here also of but little importance at the present day if the examiner is able to exclude pregnancy, phantom tumor, and fat. Operation is indicated in practically all morbid growths of the abdomen, with the exception of inoperable malignant disease; no surgeon should undertake any abdominal operation unless he is prepared to deal with any condition that may be found. The difficulty of making a differential diagnosis is well illustrated by many cases that have been recorded, in which it was impossible to determine the true nature of the tumor even after the abdomen had been opened. It is of the greatest importance to exclude pregnancy. Many women have been subjected to the operation of celiotomy because the pregnant uterus was mistaken for an ovarian tumor. Women themselves often intentionally mislead the physician, especially if the pregnancy is illegitimate. They will even carry the deception so far as to go upon the operating table with the full knowledge that they have deceived the surgeon as to their condition. The physician should always remember the possibility of pregnancy in examining any form of abdominal tumor in women. The mistakes that have happened have usually been the result of carelessness or ignorance on the part of the physician, though some of the most experienced operators have made this error. The separation of the uterus by bimanual examination as distinct from the abdominal tumor is the most valuable point in the differential diagnosis. The complication of pregnancy with an ovarian cyst renders the diagnosis more difficult. It is easier to make a differential diagnosis between an ovarian cyst and pregnancy than between some forms of uterine fibroid and pregnancy. Repeated examinations are often necessary. It is always advisable, in any case, to make two or more examinations before subjecting the woman to operation. Much which was not at first apparent may be learned by several days of watching and repeated examination. _Phantom tumor_ is a rare condition. A woman imagines that she is suffering from a tumor and that her abdomen is increasing in size. The condition is likely to occur at the menopause, and there may readily be some physical grounds for the woman’s suspicions, because there may be a constantly increasing accumulation of fat in the abdominal walls and the omentum.