Economic evaluations, such as cost-offset analyses, are receiving increased attention by US health insurers and payers. Administrative data collected by health maintenance organizations (HMOs) are considered an efficient source of utilization and cost measures for multisite economic analyses that increase external validity. However, pooling administrative data are problematic because HMO data sources reflect differences in systems of care, costing and coding. This paper describes issues arising from the pooling of HMO administrative cost data for use in multisite economic evaluations. We describe the attributes of administrative data that are relevant to costing and discuss their implications. We then briefly describe our experience with pooling these data, discuss lessons learned and offer suggestions for researchers working with such data.