A 77-year-old man presented with ascites which was due to hypothyroidism. After the ascites had been brought into remission by salt restriction, diuretics and paracentesis, monotherapy with levothyroxine prevented recurrence. Ascites reappeared as soon as levothyroxine was withdrawn due to the patient's non-compliance. Distinctive aspects of this type of ascites are the high concentration of total protein in the ascites and the high serum-to-ascites albumin gradient (SAAG). The pathogenesis of ascites in hypothyroidism is unknown. Possible hypotheses include increased capillary permeability, obstruction to lymphatic flow caused by hyaluronic acid-albumin complexes, and diminished water diuresis due to excess antidiuretic hormone. A patient with unexplained ascites should be tested for hypothyroidism, especially when the SAAG is high (> 11 g/l).