Herpes simplex virus (HSV) was recovered from the cerebrospinal fluids (CSF) of 10 patients with acute aseptic meningitis. HSV type 1 was isolated from the CSF of a 6-month-old boy. The other 9 HSV isolates from CSF were from adults in the age-group 15 to 29 years; 7 of these isolates were available for typing and were identified as HSV type 2. In a further 9 patients with acute aseptic meningitis and belonging to the same age-group a HSV infection was suggested by a significant titre rise in complement-fixing antibodies against HSV. The clinical features of the 10 patients with HSV isolation from CSF are presented. The 6-month-old boy with HSV type 1 had an acute benign aseptic meningitis. Seven of the 9 adults with HSV isolation from CSF (type 2 in all isolates available for typing) had an acute uncomplicated aseptic meningitis, one patient (with type 2 isolated) had an acute meningoencephalitis and one (isolate untyped) an acute meningomyelitis.