Objective To determine the clinical effectiveness of self management compared with routine care in patients on long term oral anticoagulants. Design Multicentre open randomised controlled trial. Setting Midlands region of the UK. Participants 617 patients aged over 18 and receiving warfarin randomised to intervention (n = 337) and routine care (n = 280) Front 2470 invited; 193/337 (57%) completed the 12 month intervention. Intervention Intervention patients used a point of care device to measure international normalised ratio twice a week mid a simple closing chart to interpret their dose of warfarin. Main outcome measure Percentage of time spent within the therapeutic range of international normalised ratio. Results No significant differences were found in percentage of time in the therapeutic range between self management and routine care (70% v 68%). Self managed patients with poor control before the study showed an improvement in control that was not seen in die routine care group. Nine patients (2.8/100 patient years) had serious adverse events in the self managed group, compared with seven (2.7/100 patient years) in the routine care arm (chi(2)(df = 1) = 0.02, P = 0.89). Conclusion With appropriate training, self management is safe and reliable for a sizeable proportion of patients receiving oral anticoagulation treatment. It may improve die time spent within the therapeutic range for patients with initially poor control.