BACKGROUND: Adverse drug reactions (ADRs) are a frequent cause of mortality and morbidity to patients worldwide, with great associated costs to the healthcare providers including the NHS in England. We examined trends in hospital admissions associated with adverse drug reaction in English hospitals and the accuracy of national reporting. METHODS: Data from the Hospital Episode Statistics database (collected by the Department of Health) was obtained and analysed for all English hospital episodes (1998-2005) using ICD-10 codes with a primary (codes including the words ('drug-induced' or 'due to') or secondary diagnosis of ADR (Y40-59). More detailed analysis was performed for the year 2004-2005 RESULTS: Between 1998 and 2005 there were 447 071 ADRs representing 0.50% of total hospital episodes and over this period the number of ADRs increased by 45%. All ADRs with an external code increased over this period. In 2005 the total number of episodes (all age groups) was 13,706,765 of which 76,692 (0.56%) were drug related. Systemic agents, which include anti-neoplastic drugs, were the most implicated class (15.7%), followed by analgesics (11.7%) and cardiovascular drugs (10.1%). There has been a 6 fold increase in nephropathy secondary to drugs and a 65% decline in drug induced extra-pyramidal side effects. 59% of cases involving adverse drug reactions involved patients above 60 years of age. CONCLUSION: ADRs have major public health and economic implications. Our data suggest that national Hospital Episode Statistics in England have recognised limitations and that consequently, admissions associated with adverse drug reactions continue to be under-recorded. External causes of ADR have increased at a greater rate than the increase in total hospital admissions. Improved and more detailed reporting combined with educational interventions to improve the recording of ADRs are needed to accurately monitor the morbidity caused by ADRs and to meaningfully evaluate national initiatives to reduce adverse drug reactions.