Appreciating how the propensity to be immunized against the flu depends on individual characteristics and environments is essential for policies regarding influenza control to be formulated sensibly. To this point, the literature has offered little documentation on the determinants of influenza immunization. Beyond epidemiology, there are important economic issues that must be addressed to understand this form of preventive care. One concerns the relationship between labour supply and immunization propensity: While it is relatively costly (in terms of time costs) for workers to obtain immunizations, workers also have relatively more to lose from being ill with the flu. Another concern not generally appreciated is the extent to which individuals' perceived risks of infection may affect their propensities to be immunized. The paper also attempts to shed light on these issues. The analysis uses data from the 1991 National Health Interview Survey. Immunization propensity displays expected patterns by age and health status, while the results with respect to race, household structure, income and insurance are somewhat more surprising and/or novel. The estimated labour supply and perceived risk effects suggest that some aspects of the economics of preventive care generally not considered in empirical work are important and merit further consideration.