We sought to study the phenomenon of patients having access to their own medical records in order to determine the impact on them and on their relationship with their health care providers. We created the Patient Clinical Information System (PatCIS) to interface with the clinical data repository at New York Presbyterian Hospital to allow patients to add to and review their medical data. We also provided educational resources and automated advice programs. We provided access to the system to thirteen subjects over a nineteen-month period and reviewed their activities in the system's usage log. We also collected data via questionnaire and telephone interview. We found that patients varied in their use of the system, from once a month or less to one or more times per day. All patients primarily used the system to review laboratory results. Both they and their physicians believed that use of the system enhanced the patients' understanding of their conditions and improved their communication with their physicians. There were no adverse events encountered during the study.