Eighteen patients with periventricular leukomalacia (PVL) of the focal or widespread type were immunohistochemically studied with respect to myelination and ferritin-containing oligodendrocytes and compared with 23 controls. Morphometric examination of myelination and ferritin-containing oligodendrocytes revealed normal development in the nonnecrotic cerebral white matter of the focal PVL brains. Myelination was mainly impaired in the necrotic or gliotic periventricular white matter of the widespread PVL brains. The expression of lipid components was poorer than that of myelin basic protein and the number of ferritin-containing oligodendrocytes was decreased in the necrotic or diffuse gliotic region of the widespread PVL brains compared to the controls. There was a significant relationship between the number of ferritin-containing oligodendrocytes and the degree of myelination. The impaired myelination in the PVL brains occurred in the necrotic as well as gliotic regions in the cerebral white matter, and may be related to the decrease of normal oligodendrocytes.