In Korea, a large-scale IEEE 802.11b-based public wireless LAN (WLAN) service, called NESPOT, has been in operation by Korea Telecom (KT) across the country during the last four years. Along with a fast growth of the service, however, the service quality problems have been encountered. In order to manage and overcome such problems properly, we have developed a diagnostic tool, which is composed of a database for wireless connection-related dissatisfaction, a log collector and analyzer, and a mechanism to report breakdown of access points (APs). To our best knowledge, this is the first attempt to address an integrated management and develop a diagnosis architecture for large-scale commercial WLANs.