Central nervous system (CNS) remyelination has been found by light and electron microscopy and immunocytochemistry within fresh lesions from four cases of acute and chronic progressive multiple sclerosis (MS). The remyelination was sparse and existed in an edematous parenchyma containing oligodendrocytes, macrophages, hypertrophic astrocytes and inflammatory components, but no fibrillary astrogliosis. It is suggested that this type of CNS remyelination in MS is a transient event. Possible underlying factors are discussed.