BACKGROUND: Warfarin is involved in the majority of fatal adverse drug events in Norway. The aim of this study is to identify risk factors behind the haemorrhagic complications. MATERIAL AND METHODS: We analysed all adverse event reports involving bleeding related to warfarin that were received by the Norwegian Medicines Agency from 1990 to 2000. RESULTS: 713 reports were included; 71% of the patients were above 70 years of age. The most frequent diagnosis was atrial fibrillation (39%). Cerebral bleedings were reported in 57% of the cases, 73% of which were fatal, as were 39 % of gastrointestinal bleedings and 14% of other bleedings. International normalised ratio values (INR values) at the time of bleeding were reported in 83% of the cases; mean INR value was 4.4 (range 1.2 - > 8.0). INR values above recommended limits at the time of bleeding were found in 74% of the patients. In 63%, bleedings occurred during the first month; in 30% during the first five days. Median duration of treatment was shorter in fatal (16 days) than in non-fatal cases (24 days). INTERPRETATION: Our results show that haemorrhagic complications are associated with high INR values and initiation of treatment. Simple strategies for reducing bleedings include better monitoring of patients, careful dose adjustment, and INR values in the lower end of the recommended ranges.