"Background: Potentially inappropriate medications (PIMs) are often found in high proportion among the elderly population. The STOPP criteria have been suggested to detect more PIMs in European elderly than the Beers criteria. Objective: This study aimed to determine the prevalence of PIMs and potential prescribing omissions (PPOs) in a sample of Portuguese nursing homes residents. Setting: Four elderly facilities in mainland Portugal Method: A descriptive cross-sectional study was used. Elderly polypharmacy patients were included in the study and their medication (registered in patient clinical records) analysed using the Beers (2012 original version and 2008 version adapted to Portugal), STOPP (Screening Tool of Older Person's Prescriptions) and START (Screening Tool to Alert doctors to Right Treatment) criteria. Data were analysed using univariate and bivariate descriptive statistics, considering a confidence interval of 95 %. Main outcomeE measures: Prevalence of PIMs and PPOs. Results: The sample included 161 individuals, with a mean age of 84.7 years (SD = 6.35), 68.9 % being female. A total of 807 PIMs and 90 PPOs were identified through the application of the three set of criteria. The prevalence of PIMs using the most recent version of the Beers criteria was 85.1 and 42.1 % for independent and dependent of diagnosis, respectively. The Portuguese adaptation of this same tool indicated a lower prevalence of PIMs, 60.3 and 16.7 %, respectively. The prevalence of PIMs using the STOPP criteria was 75.4 %, whilst the prevalence of PPOs, using START, was 42.9 %. There were significant differences in the mean number of PIMs detected depending on the tool used. (p