This paper discusses the complications associated with herpes zoster, with emphasis on its neurological manifestations. These complications, which are particularly prevalent in elderly and immunodeficient patients, include focal muscle paralysis, contralateral hemiplegia, myelitis, cranial nerve palsies and meningoencephalitis. A causative relationship with herpes zoster in many of these syndromes is probably more common than previously suspected due to difficulties in diagnosis and lack of awareness among clinicians. Zoster sine herpete-- reactivation of varicella zoster virus without rash--is associated with a spectrum of neurological disease and, for obvious reasons, is particularly difficult to diagnose. The polymerase chain reaction could be a valuable tool in overcoming these diagnostic problems, especially in patients without characteristic eruptions, allowing the early initiation of effective antiviral therapy.