Ascites is the abnormal build-up of fluid in the abdomen. Technically, it is more than 25 ml of fluid in the peritoneal cavity, although volumes greater than one liter may occur. Symptoms may include increased abdominal size, increased weight, abdominal discomfort, and shortness of breath. Complications can include spontaneous bacterial peritonitis.

In the developed world, the most common cause is liver cirrhosis. Other causes include cancer, heart failure, tuberculosis, pancreatitis, and blockage of the hepatic vein. In cirrhosis, the underlying mechanism involves high blood pressure in the portal system and dysfunction of blood vessels. Diagnosis is typically based on an examination together with ultrasound or a CT scan. Testing the fluid can help in determining the underlying cause.

Treatment often involves a low-salt diet, medication such as diuretics, and draining the fluid. A transjugular intrahepatic portosystemic shunt (TIPS) may be placed but is associated with complications. Attempts to treat the underlying cause, such as by a liver transplant may be considered. Of those with cirrhosis, more than half develop ascites in the ten years following diagnosis. Of those in this group who develop ascites, half will die within three years. The term is from the Greek askítes meaning "baglike".