Patient Centered Access to Secure Systems Online (PCASSO) is a National Information Infrastructure research project funded by the US National Library of Medicine (NLM). PCASSO is specifically designed to address the problem of enabling secure access to health information over the Internet, not just for healthcare providers and medical researchers, but also for patients to view their own medical records. The project is using familiar Web technologies to support the search and retrieval of information, including patient demographics, medications, lab tests, and transcription reports. State-of-the-art security technologies protect patient privacy and the integrity of patient information. This paper describes the security architecture of the PCASSO system.