Pulmonary capillary hemangiomatosis (PCH) is a rare cause of pulmonary hypertension; only three cases have been reported. Four additional cases are described in this report. All of the patients in this study presented with signs and symptoms of pulmonary hypertension, and in none was a correct morphologic diagnosis made during life. Histologically, the most striking feature was the presence of numerous cytologically benign thin-walled capillary-sized blood vessels proliferating diffusely through alveolar walls and in and around larger vessels and airways. Venous infiltration was associated with intimal fibrosis and secondary veno-occlusive disease (VOD). Because of the presence of hemorrhage, the apparent interstitial widening and inflammation, and the venous changes, the condition in these cases was initially misdiagnosed as interstitial fibrosis or VOD. However, the identification of proliferating and invasive capillaries, which are unique to PCH, led to the correct diagnosis. Although the nature of PCH is unknown, it behaves like a low-grade vascular neoplasm.