Lymphangiomyomatosis, clear cell sugar tumor of lung (PEComa) and pecomatosis belong to the PEComatous family of tumors, along with renal and extrarenal angiomyolipoma, primary extrapulmonary sugar tumor, clear cell myomelanocytic tumor, and abdominopelvic sarcoma of perivascular epithelioid cells. PEComatous tumors arise in a variety of sites and are tied together through their association with the tuberous sclerosis complex and the mutations in tuberous sclerosis genes TSC1 on chromosome 9q34 and TSC2 on 16p13.3. The TSC1 protein Hamartin and the TSC2 protein Tuberin form a complex which inhibits the activity of mammalian target of rapamycin (mTOR), a kinase that regulates protein synthesis and cell growth. TSC mutations result in the absence of this inhibitory influence on mTOR and lead to cellular overgrowth and formation of tumors. Patients with lymphangiomyomatosis involving lungs often develop chylous pleural effusions. The image shows reactive mesothelial hyperplasia secondary to repeated chylothorax.