The aim of our study was to determine factors influencing occurrence of haemorrhagic events during acenocumarol therapy. A total of one hundred and forty patients were interviewed using a questionnaire. From one hundred forty (100%) of interviewed patients, forty one (29.3%) had haemorrhagic events and in eighteen (12.9%) cases the event followed the use of nonsteroid anti-inflammatory drugs (NSAID's) or paracetamol in combination with acenocumarol (AC). We found eleven cases of major bleeding in eight (5.7%) of patients Eight cases of bleeding were followed the use of NSAID's or paracetamol in combination with (AC). We found that there is a statistical relationship between the higher frequency of INR examination (at least as every four weeks) and decreased occurrence of haemorrhagic events (p<0.05). The frequency of INR examination should be done once every four weeks or more often, if there is evidence to suggest that testing more frequently. Monitoring of INR with frequency less than once in 4 weeks may increase the risk of bleeding in those patients. Concomitant use of NSAID's or paracetamol in combination with acenocumarol in patients without medical consultation can be associated with growing number of bleeding. Patients on AC therapy need to be inform about frequency of INR examination and acaenocumarol interactions with other medications, which are available without the prescription, to avoid haemorrhagic events.