Varicella zoster virus is highly contagious and can cause serious complications in immunocompromised patients. To prevent people exposed to the virus from developing secondary varicella we have used oral aciclovir as post-exposure prophylaxis (PEP) since 2000. Between 2000 and 2007, there were 11 unexpected occurrences of varicella and 11 unexpected occurrences of zoster in our paediatric wards. There were 174 contacts, 131 exposed to varicella and 43 exposed to zoster. A total of 163 (94%) received PEP and 11 (6%) did not. The rates of secondary infection among contacts given prophylaxis with aciclovir only were 2.1% (3/141) for all contacts and 1.3% (1/76) for immunocompetent contacts. The rate of secondary infection among contacts not given PEP was significantly higher (18%, 2/11) (P<0.05). No adverse events due to PEP were reported. We conclude that oral aciclovir PEP following exposure to VZV on paediatric wards is both safe and effective.