Warfarin, the standard oral anticoagulant drug used in Ireland, is a widely prescribed medication, particularly in the elderly. A HSE Mid-Western Area wide audit was undertaken over a 12-month period to examine the prevalence and indications for warfarin use and haemorrhagic complications associated with the drug. Every patient receiving warfarin therapy over a 13-week period was included (2564). The age standardised rate varied from 0.09% of 35-39 year olds to 6.1% of 80-84 year olds. Atrial fibrillation was the most common indication (54%) in patients attending the Mid-Western Regional Hospital anticoagulation clinic. The annual cumulative incidence of adverse haemorrhagic events in patients with a recorded INR > or = 5.0 episode was 16.6%. The incidence of major and minor haemorrhagic events per INR > or = 5.0 episode was 1.3% and 15.3% respectively. The most common sites of haemorrhage were genitourinary (39%) and gastrointestinal (27%). No fatal or intracranial haemorrhage relating to episodes of over-anticoagulation were reported during the audit period. The most frequent reason for over-anticoagulation was drug interaction (43%). In 74% of patients, the elevated INR was reversed by omitting or reducing warfarin dose. In 17% of cases, vitamin K was administered. Only 3% of incidents were treated with fresh frozen plasma or prothrombin complex concentrates.