A 32-year-old primigravida presented with cough and dyspnea at 16 weeks' gestation. Chest roentgenogram revealed a large pleural effusion and diffuse interstitial infiltrates. Moderate arterial hypoxemia and a significant reduction in vital capacity were present. Thoracentesis revealed sterile chyle with no evidence of malignancy. Spontaneous delivery of a healthy infant occurred at 38 weeks, but no change was seen in either the pulmonary infiltrates or chylothorax. Open lung biopsy confirmed the clinical impression of pulmonary lymphangiomyomatosis, and a pleurodesis was performed. Progesterone and estrogen receptor assays on the lung biopsy material revealed only minimal binding. Following two years of therapy with tamoxifen citrate and megestrol acetate, the chylothorax has not recurred, and there has been no other appreciable change in pulmonary function.