The lipoprotein electrophoregrams and the cholesterol and triglyceride levels of the pleural fluid were evaluated for patients with chylous pleural effusions, as defined by the presence of a distinctive band of chylomicrons on the lipoprotein electrophoregram, and in patients with nonchylous effusions of various causes. One hundred forty-one patients were studied during a 3-year period. The chylous effusions had strikingly higher triglyceride levels (median 249, range 49 to 2,270 mg/dl) than the nonchylous group (median 33, range 13 to 107 mg/dl); there were no significant differences in cholesterol or protein between the two groups. The gross description of the fluid was a poor indicator of its origin, being described as consistent with chyle in less than 50% of cases of chylous effusions. The triglyceride values distinguished chylous effusion from nonchylous effusion; values greater than 110 mg/dl are highly suggestive of a chylous effusion. Equivocal cases--triglyceride values between 50 and 110 mg/dl--required lipoprotein analysis. Pleural effusions of undetermined cause, regardless of gross appearance of the fluid, require that a screening triglyceride value be obtained to rule out a chylous effusion.