AIM: To identify the possible factors determining the dose of warfarin prescribed in patients receiving anticoagulant treatment. METHODS: The computerised records of 2305 patients maintained on the drug in seven hospitals were amalgamated and classified into one of seven diagnostic groups. The associations with the dose of warfarin prescribed were investigated by univariate and multiple regression analysis. Differences between hospitals were studied with regard to the coagulometric method and the thromboplastin preparation used. RESULTS: The geometric mean dose of warfarin was 4.57 mg and 5% of patients were prescribed 10 mg or greater. There was a noticeable decrease in dose with increasing age, which averaged about 6 mg for patients aged 30 but 3.5 mg for those aged 80. Men required slightly more warfarin than women. Patients with heart disease or atrial fibrillation required lower doses of warfarin, while higher doses were required by patients with deep vein thrombosis. Significant differences in mean warfarin dose among the seven hospitals were evident. These differences could not be explained entirely by the use of different coagulometric methods or thromboplastins. CONCLUSIONS: Clinicians should be aware that older patients need reduced doses of warfarin. The considerable differences in doses of warfarin among hospitals indicates that further efforts to improve uniformity are required.