BACKGROUND: Inappropriate prescribing encompasses acts of commission i.e. giving drugs that are contraindicated or unsuitable, and acts of omission i.e. failure to prescribe drugs when indicated due to ignorance of evidence base or other irrational basis e.g. ageism. There are considerable published data on the prevalence of inappropriate prescribing; however, there are no recent published data on the prevalence of acts of omission. The aim of this study was to calculate the prevalence of acts of prescribing omission in a population of consecutively hospitalised elderly people. METHODS: A screening tool (screening tool to alert doctors to the right treatment acronym, START), devised from evidence-based prescribing indicators and arranged according to physiological systems was prepared and validated for identifying prescribing omissions in older adults. Data on active medical problems and prescribed medicines were collected in 600 consecutive elderly patients admitted from the community with acute illness to a teaching hospital. On identification of an omitted medication, the patient's medical records were studied to look for a valid reason for the prescribing omission. RESULTS: Using the START list, we found one or more prescribing omissions in 57.9% of patients. In order of prevalence, the most common prescribing omissions were: statins in atherosclerotic disease (26%), warfarin in chronic atrial fibrillation (9.5%), anti-platelet therapy in arterial disease (7.3%) and calcium/vitamin D supplementation in symptomatic osteoporosis (6%). CONCLUSION: Failure to prescribe appropriate medicines is a highly prevalent problem among older people presenting to hospital with acute illness. A validated screening tool (START) is one method of systematically identifying appropriate omitted medicines in clinical practice.