OBJECTIVE: the study aimed to determine the prevalence of and risk factors for inappropriate prescribing (IP) and prescribing omission (PO) in elderly with mental co-morbidities. PARTICIPANTS: One hundred fifty consecutive inpatients with mental co-morbidities hospitalised for acute medical illness (mean age 80 +/- 9, 70% of women) were considered for the study. MEASUREMENTS: IP and PO were prospectively identified according to STOPP/START criteria at hospital admission. RESULTS: over 95% were taking ≥ 1 medication (median = 7) which amounted to 1,137 prescriptions. The prevalence of IP was 77% and PO was 65%. The most frequent encountered IP concerned drugs adversely affecting fallers (25%) and antiaggregants therapy without atherosclerosis (14%). PO concerned antidepressants with moderate/severe depression (20%) and calcium-vitamin D supplementation (18%). Independent predictors for IP were increased number of concomitant drugs (odds ratio [OR] 1.54, 95% confidence interval [CI] 1.13-1.89), being cognitively impaired (OR 1.83, 95% CI 1.55-2.24), and having fallen in the preceding 3 months (OR 2.03, 95% CI 1.52-2.61) or hospitalised in the preceding year (OR 1.09, 95% CI 1.02-1.23). Concerning PO, psychiatric disorder (OR 1.64, 95% CI 1.42-2.01) and increase level of co-morbidities (OR 1.79, 95% CI 1.48-1.99) were identified. Living in an institutional setting was a predictive maker for both IP (OR 1.45, 95% CI 1.27-1.74) and PO (OR 1.67, 95% CI 1.32-1.91). CONCLUSION: IP and PO were highly prevalent raising the need of a greater health literacy concerning geriatric conditions in non-geriatrician practitioners who care elderly as well as in the community, in hospital and institutional settings for improving quality and safety in prescribing medication.