We developed a Web based system for ambulatory monitoring of pulmonary function and tested it in a home setting in 14 asthma patients. The system uses a portable spirometer for patient self testing and a palmtop computer that transmits the results from patients' homes to a hospital information system via phone or wireless network. The results are immediately available to physicians on a Web site, accessible via an Internet Web browser from any location. The system provides daily monitoring of 29 indices of Forced Vital Capacity test (FVC), the flow-volume loop and symptom scores. It generates alerts based on changes in data trends and exchanges messages between patients and physicians. All patients successfully uploaded their FVC data including the flow-volume loop and symptom scores from their homes to our central repository. The flow-volume loop and FVC parameters provided a more accurate assessment of the patients' asthma, and uncovered previously unrecognized nocturnal asthma and small airway obstruction. We believe our approach will be useful for asthma management on a day-To-day basis and in clinical research.