BACKGROUND: Experience from other fields of internal medicine shows that Internet-based technology can be used to monitor various diseases. The new technology handles complex calculation programs easily, and it is a unique way of communicating. These advantages might be used in optimizing the treatment for asthmatic subjects because undertreatment is a common problem found in European asthmatic subjects. OBJECTIVE: We sought to investigate the outcome of monitoring and treatment using a physician-managed online interactive asthma monitoring tool and to assess whether the outcome differs from that of monitoring and treatment in an outpatient respiratory clinic or in primary care. METHODS: Three hundred asthmatic subjects were randomized to 3 parallel groups in a 6-month prospective study: (1) Internet-based monitoring (n = 100); (2) specialist monitoring (n = 100); and (3) general practitioner (GP) monitoring (n = 100). All the patients were examined on entry into the study and after 6 months of treatment. RESULTS: The treatment and monitoring with the Internet-based management tool lead to significantly better improvement in the Internet group than in the other 2 groups regarding asthma symptoms (Internet vs specialist: odds ratio of 2.64, P = .002; Internet vs GP: odds ratio of 3.26; P < .001), quality of life (Internet vs specialist: odds ratio of 2.21, P = .03; Internet vs GP: odds ratio of 2.10, P = .04), lung function (Internet vs specialist: odds ratio of 3.26, P = .002; Internet vs GP: odds ratio of 4.86, P < .001), and airway responsiveness (Internet vs GP: odds ratio of 3.06, P = .02). CONCLUSION: When physicians and patients used an interactive Internet-based asthma monitoring tool, better asthma control was achieved.