Five cases of sudden unexpected death from primary pulmonary hypertension (PPH) are reviewed. Right ventricular myocardial hypertrophy and a dilated pulmonary conus may be the only findings at the initial gross examination. Characteristic microscopic changes in the lungs are primarily those of plexiform vascular lesions. The pathophysiology and morphologic alterations of PPH are discussed, and the association of this entity with collagen vascular diseases and mixed connective tissue disease is emphasized. It is concluded that PPH is an unusual cause of sudden unexpected death and its diagnosis may be difficult because prior signs or symptoms may be absent. The increased familial incidence of PPH underscores the necessity of considering this entity in cases of sudden unexpected death of obscure cause.