Zwoegerziekte is a disease in Dutch sheep which morphologically closely resembles an Icelandic sheep disease, Maedi 2 . Chronic lung affection and emaciation are the principal symptoms of both diseases. Visna is another chronic condition in Icelandic sheep affecting mainly the C.N.S. A virus has been isolated from sheep affected with Maedi or Visna and a close relationship in properties has been found between both viruses. In two Dutch sheep autopsied in the final stage of Zwoegerziekte, lesions were present in the C.N.S., which histologically resembled those in Visna. Another sheep slaughtered at the municipal abattoir, with lung lesions characteristic for the terminal stage of Zwoegerziekte, exhibited early cerebral lesions resembling Visna. The lesions in the C.N.S. of these three sheep were meningitis and severe encephalomyelitis of the glial type affecting the white matter. Secondary demyelination was striking. Though Zwoegerziekte has been known for several decades in the Netherlands, the accompanying C.N.S. lesions are described for the first time.