The authors describe the case of a 66-year-old female patient after transplantation of the kidney where a rare complication of immunosuppressive treatment was diagnosed--progressive multifocal leukoencephalopathy. The disease was manifested by gradually developing hemiparesis of the lower extremity five months after transplantation. Examination of the brain by computed tomography and examination of cerebrospinal fluid were normal. Subsequently the neurological finding developed into quadruparesis. Imaging of the brain by magnetic resonance revealed multiple demyelinization foci. In brain biopsy there were typical structures of progressive multifocal leukoencephalopathy and positive hybridization in situ for JC-virus in cell nuclei. Immunosuppressive treatment was restricted and ganciclovirus administration was started. Further progression of the clinical symptomatology ceased, the function of the transplanted kidney remained satisfactory. The patient died 14 months after the transplantation from sepsis. Examination of the post-mortem material revealed positivity of the JC-virus only in the cytoplasm of the affected cells. As effective treatment of infection with JC-virus is not known so far, the possible action of ganciclovirus is only speculative.