The number of patients taking warfarin is increasing. Each of these patients requires regular blood tests to ensure that the level of anticoagulation reaches and remains within a defined target range. INR monitoring is expensive and time-consuming and the frequency of the tests has repercussions for NHS resources and patient convenience. Guidelines regarding this are available, but the research on which these are based is extremely limited. Many centres in the UK are using extended intervals between tests that exceed the current guidelines. This study shows that for selected patients the interval between INR tests can safely be extended to 14 weeks with the potential for longer intervals.