Background:Although some hazards are recognised, in general, bone marrow aspiration and trephine biopsy are thought to be safe procedures. Until recently, no attempt had been made to quantify any attendant risks. For this reason, documentation of adverse events was begun in 2001, under the auspices of the British Society for Haematology. Three consecutive years have now been surveyed, the results for 2003 being presented here and compared with earlier results.Methods:Members of the British Society of Haematology were requested to document adverse events associated with diagnostic bone marrow aspirates and trephine biopsies between 1 January and 31 December, 2003. Data were collected early in 2004.Results:In total, 19 259 procedures were reported from 63 hospitals, 13 147 being combined procedures and 6112 aspirates without a trephine biopsy. Sixteen adverse events were reported, representing 0.08% of total reported procedures. The major adverse event was haemorrhage, which comprised 11 of the 16 adverse events. Although infrequent, adverse events were associated with significant morbidity and three were judged as very serious. The major risk factors for haemorrhage, in order of frequency, were diagnosis of a myeloproliferative disorder, aspirin treatment, other putative platelet dysfunctions, and thrombocytopenia.Conclusions:Adverse events following trephine biopsies and bone marrow aspirates are rare, but nevertheless can have considerable impact on individual patients.