Background: Delirium is a common underdiagnosed and undertreated problem in elderly inpatients, associated to higher morbidity, mortality and health cost. Aim: To evaluate the prevalence of delirium as hospital admission in medically ill elderly patients and the attending physician's diagnosis and treatment of delirium. Patients and methods: In a prospective and descriptive study, consecutive patients aged 65 years or more, admitted to an internal medicine ward were evaluated by independent physicians, during the first 48 h of admission, to asses the presence of delirium. Diagnosis of delirium was based on the Confusion Assessment Method. Medical and nurse records were reviewed. Family was interviewed when necessary. Results: One hundred eight patients (52% women, age range 65-94 years) with an APACHE II score of 11.6+/-5, were evaluated. Fifty seven patients (52% women, age range 65-94 years) with an APACHE II score of 5% mixed). Delirium prevalence was significantly higher in older patients (66% among those aged 75 years or older versus 30% in younger; p <0.05) and among patients with more severe conditions (88% among those with an APACHE score over 16 versus 47% below that value, p <0.05). Medical records of patients with delirium showed that this diagnosis was present only in 32% and cognitive deficit was described in 73%. Ten percent of patients with delirium received sedative medication and 38% were physically restricted. There were no environmental interventions to prevent or control delirium. Conclusions: Delirium in elderly inpatients at this until is an extraordinarily prevalent problem, seriously under diagnosed (68%) and under treated. This study should alerts our medical community to improve the diagnosis and management of delirium in elderly inpatients.