To investigate whether knowledge and perceptions of antithrombotic therapy differ between ethnic groups in the UK, we conducted a cross-sectional questionnaire survey of patients attending anticoagulation clinics in three Birmingham teaching hospitals. 180 consecutive patients were recruited-135 white European, 29 Indo-Asian, 16 Afro-Caribbean. The average knowledge score was 5.5 out of 9, with no significant differences between the groups. Indo-Asians were significantly less likely than the other groups to know the name of the anticoagulant they were taking (warfarin) and Afro-Caribbeans to know the condition for which they were being anticoagulated. Few patients of any group were able to specify more than one side-effect of warfarin or the dose they were on. In logistic regression analysis the factors associated with a low score were age >61 years, having been born outside the UK, and the perception of difficulty in comprehension. Nearly half the Indo-Asians felt unable to understand what was said to them in the clinic, and 62% expressed a preference for a doctor of the same ethnic group. Although there were no significant between-group differences, this study points to gaps in the knowledge of patients from ethnic minorities and to deficiencies in the provision of information. In patient education, these groups should receive special attention.