A case of massive postoperative ascites in a woman treated for endometrial cancer is reported. A workup for typical causes of ascites yielded negative results, prompting a more detailed analysis of the patient's condition. Hypothyroidism was discovered. After correction of the hypothyroidism, the ascites slowly resolved. Since myxedema is an uncommon cause of ascites, this is usually a diagnosis of exclusion. However, hypothroidism must be ruled out to prevent unnecessary and possibly in appropriate treatments for ascites