To assess the effectiveness of a free workplace immunization program at 3M's St. Paul, Minnesota locations, we examined the difference in sick leave hours taken from November 15, 1996, through March 15, 1997, for employees who had and did not have an influenza vaccination prior to the previous year's four-month influenza season (November 15, 1995-March 15, 1996). Among the 2,622 employees who self-reported that they were not immunized in the previous year, there were, on average, 1.2 fewer hours of sick leave taken during the 1996-1997 influenza season than the comparable time period one year earlier (P < 0.05), although the exact reason for the absenteeism was not determined. In particular, we observed that female employees younger than 50 years of age with two or more children took 3.1 hours less sick leave in the year they were immunized, compared with the preceding year (P < 0.0001). Among the 895 subjects who were immunized in both years, employees took 0.7 hours more sick leave during the 1996-1997 influenza season than the previous year (P = 0.46). Based on our findings, consideration should be given to workplace immunization programs. However, we urge caution in applying a "one-size fits all" approach to any cost-savings analysis from a company-sponsored immunization program because the workplace is not a heterogeneous environment, with regard to employees' age, gender, medical history, and home environment. All of these factors may directly or indirectly contribute to the risk of acquiring influenza and any of its complications.