The records of 95 consecutive people older than 65 years and admitted to a community hospital were retrospectively reviewed to determine the prevalence of undiagnosed delirium in hospitalized elderly patients. Chart review focused on identification of patients with documented diagnosed delirium according to Diagnostic and Statistical Manual of Mental Disorders, third edition, revised (DSM-III-R) criteria and patients with unrecognized delirium. Unrecognized delirium was considered present when information contained in a patient's chart met some or all criteria for delirium as described in DSM-III-R, but the physician's progress notes failed to indicate delirium as a diagnosed clinical entity. The prevalence of diagnosed delirium was 2%. Thirty-six percent of the patients were suspected of having unrecognized delirium. The mean length of hospital stay and the rate of mortality were significantly higher for patients with suspected delirium than for non-delirious patients. The findings of this study suggest that unrecognized delirium in the hospitalized elderly may occur frequently and is associated with an increased length of hospital stay and increased mortality.