Data on discharges from acute public hospitals in Ireland is recorded in the hospital in-patient enquiry (HIPE) system; its value in describing the casemix and pattern of resource utilization following emergency medical admissions has not been examined. We wished to determine whether there was a relationship between coded diseases at the time of discharge, patterns of investigation, and length of hospital stay (LOS) in a major teaching hospital. Data relating to emergency medical patients admitted to St James' Hospital Dublin between 1st January and 31st December 2002 was recorded. Of 5038 episodes evaluated, the median LOS was 6 days (IQR 3-13); this was significantly shorter when the patient was admitted under a general medical service (p<0.0001). There was also a positive correlation between patient age and LOS (r = 0.32; p<0.0001). No test request was associated with a shortened LOS. Prolonged LOS was associated with oesophago-gastro-duodenoscopy, echocardiography, computerised tomography, magnetic resonance imaging, and abdominal ultrasonography testing. Furthermore, prolonged LOS was associated with the following disease related groups (DRG) at discharge; codes related to heart failure, respiratory system, malignancy, stroke, diabetes, psychiatry, and anaemia. We found that clinical coding using the HIPE database strongly predicted hospital LOS for acute general medical admissions. Spare bed capacity is essential if an acute hospital is to operate efficiently and at a level of risk acceptable to patients. Therefore, determining major influences on hospital LOS is valuable, to develop potential strategies to optimise efficient utilization of acute bed capacity.