A 30-year-old Japanese woman with lymphangioleiomyomatosis (LAM) developed a left chylothorax and chylous ascites. A pleuro-peritoneal communication was confirmed by a scintigram with (99)mTc-labeled macroaggregated-albumin injected into the peritoneal cavity. Video-assisted thoracic surgery revealed a protruding papillary lesion on the left diaphragm. Chyle was oozing into the pleural cavity through this lesion. Histopathological analyses demonstrated that the protrusion was a diaphragmatic LAM lesion and that LAM-associated lymphangiogenesis enabled communication between the pleural and peritoneal cavities through lymphatic vessels. This case demonstrated a new mechanism for chylous pleural effusion in LAM and illustrates the significance of LAM-associated lymphangiogenesis.