Warfarin sodium [Coumadin] is widely used since its introduction in 1954. The dosage varies from individual to individual and is subject to adjustment due to interactions with other medications and alcohol. It is therefore important for the patient to be educated fully regarding warfarin therapy as outside the narrow therapeutic range either failure to prevent thromboembolism or serious bleeding with potential fatal complications may occur. Patient comprehension of the risks of anticoagulation, tablet recognition skills and knowledge of complications of warfarin therapy were evaluated in 150 patients attending the anticoagulation clinic in a Dublin teaching hospital. The majority, 125 (83%) perceived that they had received education about the therapy. Concomitant aspirin was avoided by 125 (83%) patients, but 25 (17%), thought it safe in combination with warfarin; 33 (22%) believed that alcohol was safe in combination with warfarin. When asked about the colours and strengths of warfarin tablets, 125 (83%) patients identified the 1 mg tablet correctly. Only 105 (70%) identified the 3 mg tablet and 98 (65%) the 5 mg tablet correctly. In 42 (28%), patients could not describe their current therapy. Potential complications from over- and under-dosage with warfarin were unknown to 89 (59%) and 90 (60%) patients respectively. This study suggests that patient knowledge regarding anticoagulation therapy is not optimal. A significant group may be at risk from serious complications because of inadequate knowledge. We suggest improving patient knowledge may improve control, reduce complication and therefore reduce the burden on the health service.