A cost-benefit analysis of routine active immunization against hepatitis B virus (HBV) for 500 nursing personnel at The Children's Mercy Hospital in Kansas City, Missouri, is described. Based on outcomes of HBV infection, local charges for medical care and wages lost from illness, the authors calculate the average cost per case of HBV infection to be $1,990. Including the current system of postexposure prophylaxis and an annual attack rate of 1 percent the current annual cost of HBV infection in The Children's Mercy Hospital is $12,630. The HBV immunization program would have to continue for 15 years before its cumulative costs ($189,133) were exceeded by those of the current system ($189,450). Therefore routine immunization of nurses would not be cost effective in the near future. If, however, a subset of high-risk employees with a 2 percent attack rate is identified, or the cost of the HBV vaccine series fell from the current $103 to $27, then routine immunization would become cost beneficial within 6 years.