Background: Delirium is a neuropsychiatric disorder, triggered by medical precipitants causes. Study aims were to describe the prevalence and impact on in-hospital mortality of delirium identiﬁed through ICD-9 codes as well as evidence of neurocognitive deﬁcits demonstrated in a population of older patients admitted to acute medical wards. Methods: This was a prospective cohort multicenter study of 2521 older patients enrolled in the “Registro Politerapie SIMI (REPOSI)” during the years 2010 and 2012. The diagnosis of delirium was obtained by ICD-9 codes. Cognitive function was evaluated with the Short Blessed Test (SBT) and single SBT items were used as measures of deﬁcits in attention, orientation and memory. Combination of deﬁcits in SBT items was used as a proxy for delirium. Logistic regression was used to evaluate the association with in-hospital mortality of delirium and combined deﬁcits in SBT items.Results: Delirium was coded in 2.9%, while deﬁcits in attention, orientation, and memory were found in 35.4%, 29.7%and 77.5% of patients. Inattention and either disorientation or memory deﬁcits were found in 14.1%, while combi-nation of the 3 deﬁcits in 19.8%. Delirium, as per ICD-9 codes, was not a predictor of in-hospital mortality. In contrast, objective deﬁcits of inattention, in combination with orientation and memory disorders, were stronger predictors after adjusting for covariates.Conclusions: The documentation of delirium is poor in medical wards of Italian acute hospitals. Neurocognitive deﬁcits on objective testing (in a pattern suggestive of undiagnosed delirium) should be used to raise awareness of delirium, given their association with in-hospital mortality.