Pulmonary microvasculopathy (formerly PCH) is another rare condition characterized by localized capillary proliferation within the lung in which capillaries invade pulmonary interstitium, vessels, and, less commonly, airways (23). The distribution of PM in the lungs is usually panlobar and patchy, resembling an interstitial process at low magnification and mimicking pulmonary congestion (24). However, closer inspection shows diffuse proliferation of microvessels containing large numbers of erythrocytes (Figure 2). These microvessels can form glomeroloid tufts or nodules that may project into the lumen of veins and lymphatics and within air spaces. A distinguishing feature, which is best appreciated on reticulin staining, is the presence of microvessels on both sides of the alveolar walls. Havlik et al. (25) require the microvessels to form at least two cell layers within the proliferating lesion for diagnosis. The endothelial cells of the abnormal capillaries are cytologically bland with elongated oval nuclei, diffuse chromatin, and indistinct cytoplasm. Mitoses are not frequently seen despite the apparent proliferative nature of the condition. The abnormal proliferating capillaries extend into bronchovascular bundles; they infiltrate the walls of arterioles, arteries, venules, and veins, invading muscular walls and occluding the lumens. Also, microvascular proliferation has been seen in perineural and intraneural positions, in the pleura, and in lymph nodes. Venous occlusion by proliferating capillaries and related intimal fibrosis is distinct from the nonangiogenic occlusion of veins in POV.