Aims: To assess the effectiveness of a Hib vaccination program against X-ray defined bacterial pneumonia in children <2 years in Colombia. Methods: 389 cases of radiologically confirmed pneumonia were recruited from hospitals in Bogota and Medellin, Colombia. Two controls per case, matched on age, sex, and socio-economic level, were selected from children attending child health services at the hospitals where pneumonia cases were admitted. Results: The risk of having X-ray confirmed pneumonia decreased with each Hib dose received; the vaccine effectiveness was 47% (2-72%) among those receiving one dose; 52% for two doses received, and 55% for three doses. These effectiveness levels remained after adjusting for other factors associated with risk of pneumonia. Conclusions: This study indicates that trials may have underestimated the proportion of radiological pneumonia in the under 2s that is due to Hib. This suggests that the impact of the vaccination will be greater than expected if it can be extended to reach the poorest children, who are at the greatest risk. (C) 2004 Published by Elsevier Ltd.