We have prospectively studied 27 adult patients attending the Department of Infectious Diseases, Göteborg, Sweden, between October 1992 and October 1996 with a diagnosis of acute viral encephalitis. In addition to cerebrospinal fluid (CSF) virus isolations and antibody analyses against herpes simplex virus, cytomegalovirus, varicella zoster virus, Epstein-Barr virus (EBV), enterovirus, adenovirus, tick-borne encephalitis virus, and mycoplasma, polymerase chain reaction test (PCR) to 5 viruses from the family of human herpes viridae, and to adenovirus as well as to enterovirus were analysed in CSF. 10 patients had herpes simplex virus type-1 (HSV-1), 1 had varicella zoster virus, 1 had tick-borne encephalitis, and 2 had Influenza A infections. In 13 patients the aetiology remained unclear. Eight patients with HSV-1 encephalitis and clinical symptoms for 2-11 d before admission were PCR-positive, while 2 patients with a < or = 2 d history of disease were negative for HSV-1 DNA on admission. These 2 patients became positive for HSV-1 DNA in CSF samples taken 4 d later in 1 case and 7 d later in the other. In 4 patients with HSV-1 encephalitis, in 1 patient with Influenza A complicated by encephalitis, and in 1 patient with encephalitis of unknown origin EBV DNA was found in CSF samples during the study. The clinical significance of these findings is unclear. The study shows that HSV-1 was the most common etiological agent in patients with viral encephalitis in the Göteborg area. In spite of improved diagnostic procedures, a large proportion of patients with symptoms and laboratory findings compatible with viral encephalitis still have an unclear aetiology.