In recent years, considerable advances have been made in treating primary pulmonary hypertension (PPH). These have provided a series of therapeutic options, ranging from the oral administration of calcium channel blockers to the continuous infusion of prostacyclin and/or lung transplantation. These therapeutic advances have highlighted the need for the better understanding of etiology and pathogenesis. Among the key uncertainties, the following are defined as leading uncertainties: (1) the nature of the initiating lesion; (2) the shared pathogenetic mechanisms that culminate in the pathologic lesions of PPH; (3) the molecular genetic bases for familial PPH and for susceptibility to PPH; (4) understanding of the obliterative-proliferative occlusive process in the small muscular pulmonary arteries; and (5) redefinition of "primary" and "secondary," ie, a revised nomenclature of pulmonary hypertension. A revised classification based on etiology is presented.