Background and Objective: We have developed an expert computer system for the control of oral anticoagulation therapy, accessible by the patients via their own computer. To investigate if the weekly measurement and dosing of international normalized ratio (INR) at home using the online Internet-based system was superior to conventional treatment, we performed a randomized, controlled trial. Patients and Methods: All 669 patients in our anticoagulation clinic were asked to participate in the trial, providing that they had Internet access and could use the CoaguChek XS system. A total of 140 patients were included and randomized to (A) once weekly measurement and report online, (B) twice weekly measurement and report online, and (C) continued conventional treatment with INR measurement in the lab every 4 weeks and dose adjustment by letter. Results: Group A had 79.7% (95% CI 79.0–80.3) of time in therapeutic range (TTR), group B 80.2% (95% CI 79.4–80.9) of TTR, and group C 72.7% (95% CI 71.9–73.4) TTR. Groups A and B perform statistically significantly better than the conventional group C, with a difference of TTR of 7% points (p