Purpose: This study aims to assess inappropriate prescribing (IP) to elderly patients during the month prior to hospitalization and to compare different IP criteria. Methods: An observational, prospective and multicentric study was carried out in the internal medicine services of seven Spanish hospitals. Patients aged 75 years and older were randomly selected after hospital admission for a year. To assess potentially inappropriate medicines (PIMs), the Beers and STOPP criteria were used and to assess Potentially Prescribing Omissions (PPOs), the START criteria and ACOVE-3 medicine quality indicators were used. An analysis to assess factors associated with IP was performed. Results: 672 patients [median age (Q1_Q3) 82 (79-86) years, 55.9% female] were included. Median prescribed medicines in the month prior to hospitalization were 10(Q1-Q3 7-13). The prevalence of IP was 87.6%, and 54.3% of patients had PIMs and PPOs concurrently. A higher prevalence rate of PIMs was predicted using the STOPP criteria than with the Beers criteria (p<.001) and a higher prevalence of PPOs using the ACOVE-3 criteria than using the START criteria (p<.001) was observed. Polypharmacy (≥10 medicines) was the strongest predictor of IP [OR=11.34 95% confidence interval (CI) 4.96-25.94], PIMs [OR=14.16, 95% CI 6.44-31.12], Beers-listed PIMs [OR=8.19, 95% CI 3.01-22.28] and STOPP-listed PIMs [OR=8.21, 95% CI 3.47-19.44]. PIMs was the strongest predictor of PPOs [OR=2.79, 95% CI 1.81-4.28]. Conclusions: A high prevalence of polypharmacy and PIMs and PPOs were reported. More than half the patients had simultaneous PIMs and PPOs. The related factors to PIMs and PPOs were different.