The Delirium Observation Screening (DOS) scale, a 25-item scale, was developed to facilitate early recognition of delirium, according to the Diagnostic and Statistical Manual-IV criteria, based on nurses' observations during regular care. The scale was tested for content validity by a group of seven experts in the field of delirium. Internal consistency, predictive validity, and concurrent and construct validity were tested in two prospective studies with high risk groups of patients: geriatric medicine patients and elderly hip fracture patients. Among the patients admitted to a geriatric department (N = 82), 4 became delirious; among the elderly hip fracture patients (N = 92), 18 became delirious. The DOS scale was determined to be content valid and showed high internal consistency, alpha = 0.93 and alpha = 0.96. Predictive validity against the Diagnostic and Statistical Manual-IV diagnosis of delirium made by a geriatrician was good in both studies. Correlations of the DOS scale with the Mini Mental State Examination (MMSE) were Rs -0.79 (p < or = 0.001) in the hip fracture patients and Rs -0.66 (p < or = 0.001) in the geriatric medicine patients. Concurrent validity, as tested by comparison of the research nurse's ratings of the DOS scale and the Confusion Assessment Method (CAM), for the group of hip fracture patients was 0.63 (p < or = 0.001). Construct validity of the DOS was tested against the Informant Questionnaire of Cognitive Decline in Elderly (IQCODE), a preexisting psychiatric diagnosis and the Barthel Index. Correlation with the IQCODE was 0.74 (p < or = 0.001) in the study with the hip fracture patients and 0.33 (p < or = 0.05) in the study with the geriatric medicine patients. Correlation with the Barthel Index was -0.26 (p < or = 0.05) in the geriatric medicine patients and -0.55 (p < or = 0.001) in the hip fracture patients. The overall conclusion of these studies is that the DOS scale shows satisfactory validity and reliability, to guide early recognition of delirium by nurses' observation.