OBJECTIVE To improve the standard of managing anticoagulant treatment and provide a basis for therapeutic quality control. DESIGN Implementation of a comprehensive computerised system for decision support. SETTING Three anticoagulation clinics in South Warwickshire. SUBJECTS Patients in South Warwickshire receiving anticoagulant treatment from September 1988 to March 1989. MAIN OUTCOME MEASURE International normalised ratio was measured and recorded at each visit. RESULTS 688 Patients' visits were analysed statistically, and a 38% improvement was achieved in the results of international normalised ratios falling within the recommended therapeutic ranges of the British Society for Haematology. CONCLUSIONS The implementation of a computerised anticoagulation support system resulted in better management of patients. The system provides a basis for uniform management of treatment and a common platform for national or international trials.