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Ascites is excess fluid in the space between the membranes lining the abdomen and abdominal organs (the peritoneal cavity). This is typically caused by liver disease.
In people with liver disease, ascitic fluid leaks from the surface of the liver and intestine. Low levels of albumin in the blood that cause a change in the pressure necessary to prevent fluid exchange (osmotic pressure). This change in pressure allows fluid to seep out of the blood vessels. An increase in the pressure within the branches of the portal vein that run through liver (portal hypertension). Portal hypertension is caused by the scarring that occurs in cirrhosis. Blood that cannot flow through the liver because of the increased pressure leaks into the abdomen and causes ascites.

Ascites tends to occur in long-standing (chronic) rather than in short-lived (acute) disorders. It occurs most commonly in cirrhosis (severe scarring of the liver), especially in cirrhosis caused by alcoholism. Other liver disorders in which ascites may occur include alcoholic hepatitis without cirrhosis, chronic hepatitis, and obstruction of the hepatic vein. Ascites can also occur in nonliver diseases, such as cancer, heart failure, kidney failure, inflammation of the pancreas (pancreatitis), and tuberculosis affecting the lining of the abdominal cavity.

Small amounts of fluid in the abdomen do not usually produce symptoms. Mild ascites is hard to notice, but severe ascites leads to abdominal distension.
A proportion of chronic ascites patients also develop hepatic hydrothrorax, e.g. unilateral pleural fluid (mainly right-sided) due to liver disease, and many have para-umbilical herniations of the abdominal wall.

Several blood tests are commonly performed for ascites, including full blood count, electrolytes and renal function, liver enzymes, and glucose.
Physical examination generally enables an expert to distinguish ascites from pregnancy, intestinal gas, obesity, or ovarian tumors. Ultrasound or computed tomography scans (CT) can detect even small amounts of fluid. Laboratory analysis of fluid extracted by inserting a needle through the abdominal wall (diagnostic paracentesis) can help identify the cause of the accumulation.