All cases of neurological infections virologically verified as due to herpes simplex virus (HSV) during a 2-year period in the Helsinki University Hospital area were studied. Altogether 10 cases were divided clinically into two forms: encephalitis 5 cases, polyradiculomyelitis, meningitis and miscellaneous 5 cases. All the cases of encephalitis were treated with acyclovir. None of the severe cases of herpes radiculomyelitis received this drug, because of lack of a rapid virological diagnosis. Besides encephalitis HSV was associated with other types of severe diseases and those need attention for proper therapy.