Vaccinating children at day-care may be a cost-effective approach for improving influenza immunization rates in this high-risk group. This study uses influenza attack-rate data from a randomized, blinded, placebo-controlled clinical trial of live-attenuated influenza vaccine, trivalent in day-care centres from two consecutive influenza seasons, one with a moderate attack rate (H1N1 dominant) and one with a high attack rate (H3N2 dominant). Costs were measured in US dollars. In the moderate attack-rate season (vaccinated, 2.2%; placebo, 13.4%), vaccination saved US$ 5.47 per child in societal costs. In the high attack-rate season (vaccinated, 4.7%; placebo, 32.1%), vaccination led to a societal costs savings of US$ 144.44 per child.