Objectives: The aim of this study was to calculate the average cost of each hepatitis B, C and HIV follow-up carried out in the health personnel that have suffered an exposure to blood and body fluids and to estimate the cost for each of the different types of sources as well as to identify the items that account for the main part of the cost., Methods: A cost analysis was carried out. The post-exposure programme was modelled in a decision tree combining probabilities (percentage of each type of source in dependence of its positivity for the three viruses and immunization state of the health personnel against hepatitis B) and monetary costs (pesetas from 1994). Costs included: salaries, laboratory, chemist, energy, cleaning, telephone, medical and office equipment, amortization and lost productivity. A sensitivity analysis was carried out with the real fulfillment of the programme., Results: The average cost was 39,564 ptas. (29,750 ptas. applying the sensitivity analysis), with a range from 86,864 ptas. (source positive for the three viruses and injured subject not immunized) to 23,074 ptas. (source negative for the three viruses). If the source was hepatitis B positive, the average cost was 86,093 ptas. when the injured subject was not immunized and 53,232 ptas. if he was immunized. Serologic tests account for the main part of the cost (range from 72.8% to 87.7%)., Conclusions: High cost suggests an appropriate risk evaluation in order to avoid unnecessary follow-ups. The model used allows to know the cost of each potentially avoided episode and it could be used for any hospital in order to make an economical evaluation of new preventive devices.