Researchers who collaborate on clinical research studies from diffuse locations need a convenient, inexpensive, secure way to record and manage data. The Internet, with its World Wide Web, provides a vast network that enables researchers with diverse types of computers and operating systems anywhere in the world to log data through a common interface. Development of a Web site for scientific data collection can be organized into 10 steps, including planning the scientific database, choosing a database management software system, setting up database tables for each collaborator's variables, developing the Web site's screen layout, choosing a middleware software system to tie the database software to the Web site interface, embedding data editing and calculation routines, setting up the database on the central server computer, obtaining a unique Internet address and name for the Web site, applying security measures to the site, and training staff who enter data. Ensuring the security of an Internet database requires limiting the number of people who have access to the server, setting up the server on a stand-alone computer, requiring user-name and password authentication for server and Web site access, installing a firewall computer to prevent break-ins and block bogus information from reaching the server, verifying the identity of the server and client computers with certification from a certificate authority, encrypting information sent between server and client computers to avoid eavesdropping, establishing audit trails to record all accesses into the Web site, and educating Web site users about security techniques. When these measures are carefully undertaken, in our experience, information for scientific studies can be collected and maintained on Internet databases more efficiently and securely than through conventional systems of paper records protected by filing cabinets and locked doors. JAMA. 2000;284:1843-1849.