Over a 2 year period, we identified five HIV-infected patients who presented with central nervous system infection caused by varicella-zoster virus, three with myelitits, and two with meningoencephalitis. All five patients were profoundly immunocompromised. Clinical presentation of these patients overlapped to a significant extent with diseases caused by other viruses, e.g. CMV. Indeed, in one case, a dual infection with CMV was diagnosed, but the respective role of each virus was ascertained by in situ hybridisation. At the time of CNS involvement, only one patient had active VZV cutaneous lesions, which were instrumental in diagnosing her condition. In contrast, PCR for VZV DNA in the CSF was helpful in making a diagnosis in the four other cases, one of which was confirmed by a post mortem. Of these five patients, two patients developed VZV disease while receiving oral acyclovir and had foscarnet treatment initiated when MRI demonstrated widespread lesions. They did not respond to antiviral therapy. The three other patients had intravenous acyclovir initiated at a time when no or limited parenchymal lesions were observed by MRI. Two of these three patients had VZV infection diagnosed solely on the basis of PCR: all three responded to treatment. Our data show that reactivation of VZV involving the central nervous system occurs frequently in the absence of cutaneous lesions. PCR of cerebrospinal fluid may help in making an early diagnosis which is probably a prerequisite for successful treatment of VZV infection of the CNS.