A five-year-old spayed shorthair cat was evaluated for chylous effusion in the thorax and abdomen. Thoracic and abdominal radiographs revealed large volumes of pleural and peritoneal fluid within a ground glass appearance and obscuration of the abdominal Structures. Ultrasonographic examination confirmed the presence of pleural and peritoneal effusion. The diagnosis of chylothorax was confirmed by serum and pleural fluid cholesterol : triglyceride ratios (a ratio of less than 1 : 120/3403 mg/dL in pleural fluid and 147/570 mg/dL in serum) and lipoprotein electrophoresis with a great increase in proportion of chylomicron to 17.7%. The test for heartworm antigen was negative. The cat was tested for antibodies to FIV and FeLV antigens and was found to be FIV-positive.