PURPOSE: To assess the quality of anticoagulation control at an out-patient clinic and to investigate patterns of deviation from therapeutic ranges. DESIGN/METHODOLOGY/APPROACH: Records for 36,157 clinic visits (2050 patients) were studied. The quality of anticoagulation control was assessed by comparing the measurement of pro-thrombin time recorded at each clinic visit, expressed as an international normalised ratio (INR), with the target therapeutic range for that patient, also recorded at the time of the clinic visit. Each INR measurement was classified according to the relevant patient's therapeutic range and the signed difference between the INR measurement and the centre of the therapeutic range was calculated. For each patient the percentage of their INR measurements that lay within their therapeutic range was calculated. FINDINGS: Of the measurements, 52.3 per cent were within the relevant therapeutic range. The proportion of individual patients' INR measurements within range varied greatly (median 52 per cent, inter-quartile range 40-65 per cent). The quality of anticoagulation control, as measured by the proportion of patients within their therapeutic range, changed little with patient follow-up time. ORIGINALITY/VALUE: The quality of anticoagulation control reported is comparable with that at other centres. The vast majority of patients spend periods outside the therapeutic range for their condition. There may be considerable room for improvement.