Background. We investigated the characteristics, frequency, and prognosis of central nervous system (CNS) involvement in patients with hemophagocytic lymphohistiocytosis (HLH). Procedure. Neurological manifestations were prospectively assessed in 92 children with HLH treated from January 2004 to August 2008 at Our center; 82 (89%) had associated viral infections (69 Epstein-Barr virus), one empyema, while no associated disease was identified in the remaining nine. Prior to treatment, all underwent cerebrospinal fluid (CSF) evaluation, brain computer tomography (CT) and/or magnetic resonance imaging (MRI). Results. At diagnosis, 43 (47%) children had CNS involvement. Twelve patients (13%) had neurological symptoms, including seizures, ataxia, coma, cranial nerve palsy, and hemiplegia. All patients improved after 8 weeks of therapy, but one later developed progressive neurological symptoms and six discontinued therapy due to progressive systemic symptoms and/or other reasons. Fifteen patients had CSF abnormalities that all normalized completely after 6 weeks of treatment. Thirty-six patients (39%) had neuroradiological abnormalities; with 5 still under treatment, 15 lost to follow-up, and 16 followed after completion of therapy. Of these 16, 12 improved, 3 were unchanged, and I progressed. Among all 21 children with CNS involvement followed after completion of therapy, 10 recovered completely, 10 improved (3 had remaining neuroradiological abnormalities), and I progressed clinically and neuroradiologically. Conclusion. Most patients reported here Suffered from secondary HLH and since CNS involvement is frequent in HLH, brain MRI at diagnosis is recommended in all HLH patients. Clinical and CSF abnormalities often improved within 8 weeks of therapy, but CT/MRI abnormalities normalized more slowly and less frequently. Pediatr Blood Cancer 2010;54:408-415. (C) 2009 Wiley-Liss, Inc.