The aim of this prospective study was to determine the extra-length of stay and the average cost for rotavirus healthcare-associated infection (HAI). Children admitted to the paediatric ward of the Reims University Hospital between the 1 December 2001 and 31 March 2002, were included in a pairwise matched (1:1) case-control study. Cases were defined as patients with rotavirus HAI. Controls were selected according to matching variables in a stepwise fashion. The costs measured in this study included all expenses sustained by the hospital. Information on costs was obtained from medical records and the hospital economic department. The attack rate and the incidence of healthcare-associated acquired rotavirus infection were 6.6% and 15.8 per 1000 hospital days, respectively, during a winter outbreak. Fifteen percent of HAI were identified after discharge. The average cost per case was 1930 and the mean excess length of stay was 4.9 days. Our findings clearly demonstrate the substantial expense incurred as a result of HAI caused by rotavirus in children. To prevent these costly infections, several cost-effective measures such as standard precautions should be reinforced in the education of the healthcare workers.