A chylothorax and a cholesterol pleural effusion represent the two forms of lipid effusions encountered. Traditionally, a lipid pleural effusion is characterized by the presence of milky fluid. Although these two effusions often share a similar pleural fluid appearance due to the high lipid concentration, they have major differences in the pathogenesis, clinical presentation, diagnosis, predisposing conditions, and management of these effusions. A chylothorax is defined by the presence of chyle in the pleural space resulting from obstruction or disruption of the thoracic duct or one of its major tributaries. A triglyceride concentration > 110 mg/dL is virtually diagnostic, but the presence of chylomicrons confirms the diagnosis. However, a chylothorax defined by these criteria represents a heterogeneous group of clinical entities. The presence of chylomicrons or triglyceride levels > 110 mg/dL in a pleural effusion should be considered evidence of chyle leakage of indeterminate clinical significance. Many cases of a chylous effusion may be associated with other causes of pleural fluid formation. In the case of an acute or chronic chylothorax due to recent or remote thoracic duct injury, this assessment is essential, as surgical ligation of the thoracic duct is often entertained. In other cases, especially lymphoma or chylous ascites, treatment of the underlying condition is indicated regardless, and the assessment of the response to treatment is a reasonable initial approach. In contrast, a cholesterol effusion is typically the result of long-standing pleurisy with elevated cholesterol levels in the pleural space; however, this paradigm has been challenged. Lung entrapment with thickened parietal and visceral pleural membranes is the typical radiographic findings of a cholesterol effusion. Most cases of cholesterol pleural effusions are attributed to tuberculous or rheumatoid pleurisy. Decortication is the mainstay of treatment for a cholesterol effusion in symptomatic patients with restrictive lung function. © Thieme Medical Publishers.