AIMS: Underprescribing is increasingly recognized as an important problem. The aim of this study was to determine the relationship between polypharmacy and underprescribing. METHODS: Treatment of current medical problems in geriatric patients was compared with general practitioner and national guidelines. Underprescription was defined as lack of an indicated drug, while no reason could be found for not prescribing it. Polypharmacy was defined as five or more drugs. RESULTS: Polypharmacy was present in 61% of 150 patients. Underprescription was found in 47 (31%). Of patients with polypharmacy 42.9% were undertreated, in contrast to 13.5% of patients using four medicines or less (OR 4.8, 95% CI 2.0, 11.2). The estimated probability of underprescription increased significantly with the number of drugs. CONCLUSIONS: We found a clear relationship between polypharmacy and underprescribing.