We aimed to audit nosological inaccuracies in death certification in Northern Ireland and to compare performance of hospital doctors and general practitioners. Nosology is the branch of medicine which treats of the classification of disease. 1138 deaths were registered in Northern Ireland in a 4-week period commencing 3/10/94. 195 of these were either registered by HM Coroners (HMC) or required further investigation by their staff; these cases were excluded from the study. The remaining 943 were analysed for wording and formulation inaccuracies according to the revised notes (1974), Northern Ireland Medical Certificate of Cause of Death. These are issued in book form by the Registrar of Births and Deaths. The commonest inaccuracies in death certification occur in the areas of poor terminology, sequence errors and unqualified mode. One or more inaccuracies were found in 317 (33.6%) of cases. In 13 of these (4%) cases, the inaccuracies were serious enough to warrant referral by the Registrar of Deaths to HM Coroner. The numbers of general practitioners and hospital doctors were recorded, with general practitioners being responsible for 122 (38%) and hospital doctors being responsible for 195 (62%) of inaccuracies.