The National Water Data Exchange (NAWDEX) was developed to assist users of water-resource data in the identification, location, and acquisition of data. The Master Water Data Index (MWDI) of NAWDEX currently indexes the data collected by 423 organizations from nearly 500,000 sites throughout the United Stales. The utilization of new computer technologies permit the distribution of the MWDI to the public on compact disc. In addition, geographic information systems (GIS) are now available that can store and analyze these data in a spatial format. These recent innovations could increase access and add new capabilities to the MWDI. Before either of these technologies could be employed, however, a quality-assurance check of the MWDI needed to be performed. The MWDI resides on a mainframe computer in a tabular format. It was copied onto a workstation and converted to a GIS format. The GIS was used to identify errors in the MWDI and produce reports that summarized these errors. The summary reports were sent to the responsible contributing agencies along with instructions for submitting their corrections to the NAWDEX Program Office. The MWDI administrator received reports that summarized all of the errors identified. Of the 494,997 sites checked, 93,440 sites had at least one error (18.9 percent error rate).