INTRODUCTION: The medical and public health communities advocate immunization registries as one tool to achieve national immunization goals. Although substantial resources have been expended to establish registries across the nation- minimal research has been conducted to evaluate provider participation costs. METHODS: The objective of this study was to identify the direct costs to participate in an immunization registry. To estimate labor and equipment costs- we conducted interviews and direct observation at four sites that were participating in one of two immunization registries. We calculated mean data-entry times from direct observation of clinic personnel. RESULTS: The annual cost of participating in a registry varied extremely- ranging from \$6083 to \$24-246- with the annual cost per patient ranging from \$0.65 to \$7. 74. Annual per-patient costs were lowest in the site that used an automated data-entry interface. Of the sites requiring a separate data-entry step- costs were lowest for the site participating in the registry that provided more intensive training and had a higher proportion of the target population entered into the registry. CONCLUSIONS: Ease of registry interface- data-entry times- and target population coverage affect provider participation costs. Designing the registry to accept electronic transfers of records and to avoid duplicative data-entry tasks may decrease provider costs.