This study was based on the data collected on a consecutive sample of 106 cancer patients referred for mental status evaluation. All patients were evaluated by use of the Confusion Assessment Method (CAM) algorithm, the Delirium Rating Scale (DRS), the Memorial Delirium Assessment Scale (MDAS), and a question about the subjective perception of delirium. After comparing the diagnostic criteria of delirium on the DSM-III-R and DSM-IV, authors evaluated the ability of all DRS and MDAS items to discriminate delirium versus non-delirium patients, testing the difference in the distribution of the individual MDAS and DRS item scores. Authors also assessed the relationship between delirium diagnosis and the subjective perception of delirium. The MDAS showed a greater number of discriminating items. The items that proved to be less discriminating were "Hallucinations" and "Lability of Mood" on the DRS. Subjective perception only partially discriminated delirium from non-delirium patients. The way in which the DRS and MDAS reflected the DSM criteria are therefore partially different.