The aim of the study was to determine the incidence of viruses causing aseptic meningitis, meningoencephalitis, and encephalitis in Spain. This was a prospective study, in collaboration with 17 Spanish hospitals, including 581 cases (CSF from all and sera from 280): meningitis (340), meningoencephalitis (91), encephalitis (76), febrile syndrome (7), other neurological disorders (32), and 35 cases without clinical information. CSF were assayed by PCR for enterovirus (EV), herpesvirus (herpes simplex [HSV], varicella-zoster [VZV], cytomegalovirus [CMV], Epstein-Barr [EBV], and human herpes virus-6 [HHV-6]), mumps (MV), Toscana virus (TOSV), adenovirus (HAdV), lymphocytic choriomeningitis virus (LCMV), West Nile virus (WNV), and rabies. Serology was undertaken when methodology was available. Amongst meningitis cases, 57.1% were characterized; EV was the most frequent (76.8%), followed by VZV (10.3%) and HSV (3.1%; HSV-1: 1.6%; HSV-2: 1.0%, HSV non-typed: 0.5%). Cases due to CMV, EBV, HHV-6, MV, TOSV, HAdV, and LCMV were also detected. For meningoencephalitis, 40.7% of cases were diagnosed, HSV-1 (43.2%) and VZV (27.0%) being the most frequent agents, while cases associated with HSV-2, EV, CMV, MV, and LCMV were also detected. For encephalitis, 27.6% of cases were caused by HSV-1 (71.4%), VZV (19.1%), or EV (9.5%). Other positive neurological syndromes included cerebellitis (EV and HAdV), seizures (HSV), demyelinating disease (HSV-1 and HHV-6), myelopathy (VZV), and polyradiculoneuritis (HSV). No rabies or WNV cases were identified. EVs are the most frequent cause of meningitis, as is HSV for meningoencephalitis and encephalitis. A significant number of cases (42.9% meningitis, 59.3% meningoencephalitis, 72.4% encephalitis) still have no etiological diagnosis. J. Med. Virol. © 2012 Wiley Periodicals, Inc.