Herpes simplex encephalitis is a neurologic emergency demanding immediate institution of specific therapy in order to prevent mortality. Diagnosis, however, is a complex matter with controversy existing over the appropriateness of brain biopsy. We report the demonstration of herpes simplex virus DNA by means of the polymerase chain reaction (PRC) in the cerebrospinal fluid of 3 patients with herpes simplex encephalitis. One of the patients suffered from brain-stem encephalitis with high intensity signals in the pons on magnetic resonance imaging, the second reported case of this entity. The PCR for herpes simplex on a control series of cerebrospinal fluids of 20 patients with other central nervous system infections was negative. PCR of cerebrospinal fluid offers a sensitive, specific and rapid diagnosis of herpes simplex encephalitis, making brain biopsy unnecessary. Still, the importance of strict measures to prevent contamination cannot be stressed enough. It is possible that due to the high sensitivity of the PCR, herpes simplex may be found in other infectious syndromes of the central nervous system.