Lymphangiomyoma is an unusual disease, probably a congenital malformation or hamartoma, which is characterized by a combined proliferation of smooth muscle and lymphatic channels in the mediastinum, retroperitoneum, and regional lymph nodes, and by "honeycombing" and proliferation of smooth muscle in the lungs. Roentgenographic findings of interstitial lung disease associated with chylous pleural effusion are almost pathognomonic of this disease.