Probable progressive multifocal leukoencephalopathy (PML) was diagnosed on the basis of clinical picture and magnetic resonance imaging in a 63-year-old man with a complete remission of a non-Hodgkin's lymphoma. After the introduction of intramuscular alpha-interferon therapy, his neurological state and MRI findings showed a clear improvement. Eighteen months after the onset of first symptoms the patient has significantly recovered from both aphasia and motor impairment, and shows only a mild attentional deficit.