To determine the biochemical parameters of chylous pleural fluids and better inform current clinical practice in the diagnosis of chylothorax.We retrospectively reviewed 74 patients with chylothorax (defined by the presence of chylomicrons) who underwent evaluation during a 10-year period from January 1, 1997, through December 31, 2006. The biochemical parameters and appearance of the fluid assessed during diagnostic evaluation were analyzed.The study consisted of 37 men (50%) and 37 women (50%), with a median age of 61.5 years (range, 20-93 years). Chylothorax was caused by surgical procedures in 51%. The chylous pleural fluid appeared milky in only 44%. Pleural effusion was exudative in 64 patients (86%) and transudative in 10 patients (14%). However, pleural fluid protein and lactate dehydrogenase levels varied widely. Transudative chylothorax was present in all 4 patients with cirrhosis but was also seen with other causes. The mean +/- SD triglyceride level was 728+/-797 mg/dL, and the mean +/- SD cholesterol value was 66+/-30 mg/dL. The pleural fluid triglyceride value was less than 110 mg/dL in 10 patients (14%) with chylothorax, 2 of whom had a triglyceride value lower than 50 mg/dL.Chylothoraces may present with variable pleural fluid appearance and biochemical characteristics. Nonmilky appearance is common. Chylous effusions can be transudative, most commonly in patients with cirrhosis. Traditional triglyceride cutoff values used in excluding the presence of chylothorax may miss the diagnosis in fasting patients, particularly in the postoperative state.