Clinical and pathologic findings in seven patients who died of severe pulmonary artery hypertension due to toxic oil syndrome are assessed. These cases correspond to a late stage of evolution of the disease characterized by progressive deterioration in clinical features--increasing dyspnea, chest pain, syncope, and death (in low-output heart failure). The main pathologic pulmonary vascular findings consisted of plexiform lesions, thromboses, and venous lesions. Endothelial damage induced by the toxic agents is suggested as an initial causative mechanism, perpetuated by intimal proliferation and in situ thrombosis. Plexiform lesions appear late and active histologically. This new cause of pulmonary artery hypertension, with pathologic findings similar to those found in primary pulmonary hypertension, may help in understanding the pathophysiology of this unknown disease.