OBJECTIVES: To evaluate the impact of an influenza vaccination campaign: decrease of attack rates of influenza-like-illness (ILI), work productivity and indirect costs (potential cost savings) for the employer. METHODS: A prospective observational study was conducted among the workforce of a bank of Bogota from October 2000 to May 2001, with 2 cohorts: influenza vaccinated (volunteers) and not vaccinated. Self-administered monthly questionnaires collected socio-economic and health status information, data on ILI symptoms and sick leaves. Vaccine's adverse events were reported one week after injection. Cost-benefit analyses were performed from the employer perspective using individual operating income and salary values. Costs of vaccination included vaccine administration and adverse events. Loss of productivity was assessed by sick leave days, days of reduced effectiveness at work due to being not well because of ILI. RESULTS: Among the 759 subjects, 56% vaccinated and 44% not vaccinated, the attack rates of ILI were respectively 15% versus 47%, with presence of usual symptoms (cough, chills, muscle aches...) and fever in 93% of the reported ILI. Absence rates for ILI were similar in the 2 cohorts: 2.6%, with a mean of 3.1 days of sick leave, as well as the proportions of people feeling not well because of ILI: 93%, with a mean of 4 days before being well again. Using the realistic hypothesis of a reduced effectiveness of 30% when the patient is not well because of ILI, global cost savings were US$7469 for the study population and US$59 per individual i.e. a cost-benefit ratio of 20%. CONCLUSIONS: Among the studied volunteers, ILI has significant impact on work productivity, in terms of indirect costs, even if considering lower work effectiveness rates' hypotheses, in conditions of mild outbreak of influenza. Possible expansion of influenza vaccination to all the company could lead to important employer cost savings.