Brain biopsy has been considered the gold-standard test for the diagnosis of herpetic encephalitis. However, because of its cruence, other non invasive methods have been developed for its diagnosis. In recent years polymerase chain reaction (PCR) in cerebrospinal fluid has been employed for the diagnosis of herpetic encephalitis (HE). We report two cases of clinically suspected HE in which DNA of herpes simplex virus was amplified by PCR from cerebrospinal fluid, and review the role of this method for the diagnosis of HE. Two patients were admitted to the emergency room presenting clinical pictures and cerebrospinal fluid samples suggestive of lymphocytic meningoencephalitis. Both were admitted in the intensive care unit because of altered level of consciousness, beginning empiric treatment with intravenous acyclovir as HE was suspected. Clinical outcomes were favourable in both cases, being confirmed the presence of herpes simplex DNA in cerebrospinal fluid by PCR in the two patients. We have reviewed in the literature the value of PCR for the diagnosis of HE in comparison with other ancillary tests as brain biopsy and serology. PCR could reach according with some authors a sensibility of 98% and a specificity of 94%.