Pulmonary capillary hemangiomatosis (PCH) is an unusual disorder characterized by the proliferation of capillaries in the alveolar septa and pulmonary interstitium. Originally conceived as a primary idiopathic disorder of the pulmonary microcirculation, recent studies have demonstrated that PCH may be associated with other pathologies. Nitric oxide (NO) is a gaseous free radical with protean biological effects that is released during the intracellular conversion of arginine to citrulline. Nitric oxide synthases (NOS) mediate the production of NO and the release of NO in the microvasculature is specifically catalyzed by endothelial NOS (NOS-III). As NOS contributes to angiogenesis and is reduced in the hypertensive pulmonary microcirculation, we examined the expression of NOS-III protein in situ in the lungs of patients with PCH. Reduced microvascular expression of NOS-III protein by endothelial cells was observed in 4/6 (67%) cases of PCH, and all of these showed concomitant pulmonary vascular hypertensive remodeling. In 2/6 (33%) cases of PCH with no morphologic evidence of pulmonary hypertensive arteriopathy, endothelial expression of NOS-III protein was judged to be either minimally reduced or normal. These findings suggest that NOS-III is specifically reduced in PCH when pulmonary arterial hypertensive remodeling is concomitantly present.