Three patients with pulmonary lymphangiectasis were studied by cardiac catheterization. All had evidence of alveolar hypoventilation and pulmonary hypertension. Postmortem injection studies of the abnormal lungs demonstrated greatly dilated and intracommunicating pleural, interlobular, and perivascular lymphatics. Two of the patients had obstructed total anomalous pulmonary venous return, while the third had a ventricular septal defect. In the latter, a premortem diagnosis of pulmonary lymphangiectasis was made at 4 months of age by lung biopsy. Forty five cases from the literature have been reviewed. From the review it is postulated that pulmonary lymphangiectasis can occur in the following three forms: (1) as part of generalized lymphangiectasis; (2) secondary to pulmonary venous obstruction; and (3) as a primary developmental defect of the lung. The clinical picture and prognosis varies with the type present.