In three patients who died of immunodeficiency syndromes, including two patients with acquired immunodeficiency syndrome (AIDS), foci of necrotizing leukoencephalopathy were found in the basis pontis. The lesions were identical in location and morphology to those previously described in patients who received chemotherapy and central nervous system radiotherapy for various malignancies, and (except for their restricted anatomic location) resembled the disseminated necrotizing leukoencephalopathy that complicates central nervous system leukemia and lymphoma. The lesions are to be distinguished from central pontine myelinolysis, are confined to pontocerebellar tracts, and are not specific for the immunodeficient state, but may reflect preterminal metabolic derangements, since they seem unrelated in this clinical setting to malignancy and/or its treatment. Alternatively, they may be a consequence of the immunosuppressed state. The presence of this morphologic abnormality in two AIDS patients is especially intriguing, in view of the frequency with which white matter lesions are seen in the AIDS population.