Background of Study Ventriculostomi associated infection (VAI) in external ventricular drainage (EVD) is a well known complication in neurosurgery. It is often associated with increased mortality and morbidity, but also results in longer stay in hospital and prolonged treatment with antibiotics. The frequency of VAI differs in the literature between x and 21.1 % depending mainly on methods of diagnosis. Methods A prospective study with historical controls. From 1.1.2001 to 31.9.2007 patients was treated with standard EVD, and from 1.10.2007 -to 31.9.2008 AIS (Bactiseal, Codman ) was used. CSF-samples were collected every day. Samples was obtained from the line away from the patient and cultered on agar plates for six days in both aerobic and aerobic environment. VAI was defined as positive cultures in CSF. Results A total of 485 patients were included. No significant differences in gender, age or diagnosis. Standard drain: 417 patients. 97 patients had positive CFS-cultures (23.3 %). 10 patients had more than one infection. Most frequent findings was Propionibacterium Acnes, Coagulasenegative Staffyllococci and Staffylococcus Epidermidis accounting for 70,1 % of species. AIS-drain: 68 patients. 3 had positive CSF-culteres (4,4%) (2 Propionebacterium Acnes, 1 coagulasenegative Staffylococci) Findings was sporadic. No multiresistency was found. X2 =12,6920; p< 0,001. Conclusions AIS significantly reduces the rate of growth of bacterias in EVD. This study does not distinguish between ventriculitis, drain-infection and contamination, but deals with the everyday problem of encountering growth of bacterias in Cerebrospinal Fluid.