The diagnosis of lymphangiomyomatosis (LAM) requires characteristic radiologic findings on a high-resolution chest CT plus a diagnosis of tuberous sclerosis, renal angiomyolipoma, chylous pleural effusion, or an elevated plasma level of VEGF-D. Plasma VEGF-D levels are increased in about 60% of biopsy-proven cases of LAM and appear to correlate with disease severity. A normal VEGF-D level does not exclude LAM. In the absence of one or more of these diagnostic criteria, a biopsy becomes necessary to distinguish LAM from entities that enter into the differential diagnosis (see next slide). The image shows considerable thickening of alveolar septa due to proliferation of spindle-shaped myoid cells.