The main predictor of outcomes in herpes simplex virus (HSV) encephalitis (HSE) is the delay between hospital admission and initiation of acyclovir. In this study, factors associated with late initiation of acyclovir were identified. The study included adults from northern France whose cerebrospinal fluid (CSF) was positive for HSV by PCR. Late initiation of acyclovir was defined as a delay of >1 day from hospital admission. In total, 184 patients were retrospectively enrolled from January 1991 to December 2002. The median age was 60 years (range: 17-91), and 102 (55.4%) were male. Acyclovir was initiated >1 day after hospital admission in 68 patients (37.0%). According to multivariate analysis, independent risk factors for late initiation of acyclovir were severe underlying disease (Knaus score >or=C) (OR 4.1; 95% CI 1.5-11.7); alcohol abuse (OR 3.4; 95% CI 1.3-8.9); and a delay of >1 day from admission to first brain imaging (OR 8.4; 95% CI 3.9-18.0). In addition, univariate analysis suggested an association between a finding of <10 leukocytes/mm(3) in CSF at admission (OR 2.5; 95% CI 0.7-5.8). These characteristics were found in 26 (14.1%), 23 (12.5%), 66 (35.9%) and 27 (14.7%) patients, respectively. One risk factor was identified in 109 (59.2%) patients, two in 29 (15.8%), and three in six (3.3%). Patients with HSE often present with severe underlying disease, chronic alcohol abuse, or atypical CSF findings, and such factors should not be allowed to delay diagnosis and administration of acyclovir.