The electrocardiographic patterns encountered in 28 patients with pulmonary hypertension secondary to chronic lung disease have been studied. Thirteen had neither right ventricular hypertrophy nor right bundle-branch block; eight had either right ventricular hypertrophy alone or right ventricular hypertrophy and right bundle-branch block (complete or incomplete); and seven had incomplete right bundle-branch block. A statistical comparison is made between these various groups of the means of the following physiologic measurements: mean pulmonary artery pressure, total pulmonary resistance, arterial oxygen saturation, right ventricular work, arterial carbon dioxide content, and cardiac index. The significance of the pattern of right ventricular hypertrophy is discussed.