Progressive multifocal leucoencephalopathy is caused by infection with JC virus. The disease affects patients with immunodeficiencies, hematologic diseases, and patients treated with radio- therapy. The disease is characterised by foci of demyelinisation with atypical astrocytes and oligodendrocytes. Oligodendrocytes contain typical intranuclear inclusions. Progressive multifo- cal leucoencephalopathy and its verification is presented in three cases. Two patients died of progression of a malignant neoplasm and the leucoencephalopathy was a complication of the malignancy. The third case was a biopsy specimen taken from the brain of a patient who received a renal transplant. The material of all patients was analysed by light and electron microscopy, and in situ hybridisation with a probe specific for JC virus. In situ hybridisation proved to be the most specific and a simple method to demonstrate the infection in all cases. It is useful in instan- ces in which the histologically detectable lesion is not characteristic, and in cells in which the conventional histologic methods fail to reveal the intranuclear inclusions of JC virus.

        Key words: progressive multifocal leucoencephalopathy - in situ hybridisation