Investigating the pharmacoeconomic impact of any diagnostic or therapeutic intervention in the Irish healthcare setting is currently compromised by the lack of detailed cost data. Consequently, we conducted a number of microcosting studies in the areas of acute myocardial infarction, cardiac failure and HIV, from the hospital perspective. The results of these microcosting studies were compared with the costing estimates assigned to hospital admissions, based on the diagnosis-related group system. Differences ranged from 9 to 66%. It was concluded that the diagnosis related group system is a useful estimate of costs for patient admissions in the absence of detailed cost of illness data. However, supplementary costing studies should be performed for certain therapeutic areas, particularly those where investigation and/or treatment costs are high.