['To distinguish congenital chylothorax from other causes of hydrothorax in utero.', 'Serum and pleural fluid samples from 8 fetuses with congenital chylothorax and 5 control fetuses with other causes of hydrothorax were tested for total protein, albumin, IgG, IgA, and IgM.', 'Fetuses with congenital chylothorax had higher levels of IgG in pleural fluid, but not the other four proteins, than control fetuses (P<0.05). There were no significant differences in serum proteins among fetuses. When we examined pleural fluid to serum ratios, the IgG ratio in fetuses with congenital chylothorax was significantly higher than that of control fetuses (P<0.05). The IgG ratio in chylothorax was greater than 0.6 regardless of lymphocyte count.'] Is pleural fluid/serum immunoglobulin ratio a diagnostic marker for congenital chylothorax in utero?