Data collection is a major problem in large scale clinical studies, especially muticenter and international trials. Data collection by mailing is too slow and expensive. Furthermore, difficulty in understanding handwritten notes from many investigators could result in potential errors in data entry. We developed web-based data collection system for patterns of care study of radiation oncology to collect data nation-widely and efficiently. We introduced drop-down box, check box, and radio button in data entry form for rapid, convenient and correct data entry. Our system also checks blank item, logical errors and other validity of data in real time. For security, we committed the server to the care of Korea Internet Data Center, implemented secure socket layer for communication between web server and clients, and established two remote backup systems. Participating investigators are satisfied with this system and we are collecting data efficiently. We think that web-based data collection system should be considered in any large scale clinical studies.