Oral anti-coagulation with warfarin is increasingly required in the prophylaxis and treatment of vascular thrombosis and embolism. Unless the degree of anti-coagulation is maintained in the narrow therapeutic range either serious bleeding or failure to prevent thromboembolism may occur. Complications may occur in up to 31% of patients. We randomly sampled 50 patients attending an anticoagulant clinic and interviewed them. We found the PTR between 2.0-4.0 in 70% patients. Their records indicated that they attended 0.9 +/- 0.5 times per month, but the patients themselves said that they had 2.4 +/- 1.7 visits per month, lasting on average 1.9 +/- 0.7 hours per visit. The mean duration of therapy was 4.3 +/- 5.4 years [range 1 month to 26 years]. Many patients perceived that they had received no education about warfarin (23%) while the majority 67% of the remainder said their doctor had educated them. Concomitant aspirin was avoided by 74% patients but 14% considered it safe in combination with warfarin; 49% patients believed that alcohol was safe in combination with warfarin. When asked about the colours and strengths of warfarin tablets, 37% of our sample were completely correct, 9% were completely incorrect and 54% were partly correct. In 16% patients they could not describe their current therapy. As doctors may adjust warfarin dosage for patients in terms of tablet colour, we asked a sample of junior doctors about the colours or strengths of warfarin tablets: 10% were completely correct, one doctor knew none of the colours or strengths and the remainder had a partial knowledge. These studies suggest that the majority of patients on warfarin are cautious about therapy and are safe in their practices. However, we feel that a significant minority may be at risk from complications because of inadequate knowledge. We suggest that improving patient understanding by education may reduce complications and lead to more stable control of anticoagulant therapy.