OBJECTIVE: This study explored the phenomenology of delirium in cancer patients, and evaluated the severity of delirium according to the cancer treatment method and the use of sedative-hypnotics, including zolpidem. METHODS: This was a retrospective chart review study. Cancer patients with delirium, who were referred to the Department of Psychiatry at a National University hospital, were included in the analysis. In all subjects, the severity of delirium was assessed using the Delirium Rating Scale-Revised-98 (DRS-R-98). RESULTS: The analysis included 112 patients with delirium. Among the neuropsychiatric and behavioral symptoms, the most frequent ones were sleep-wake cycle disturbance (100.0%) and perceptual disturbances and hallucinations (95.5%). Among the cognitive symptoms, inattention (98.2%) was the most frequent. The older the patients were, the higher the DRS-R-98 total scores were. There was no significant difference in the DRS-R-98 scores among cancer treatment methods (operation, chemotherapy, or radiation therapy). However, the total DRS-R-98 scores in the zolpidem use group were higher than in control group, and the difference was significant after adjusting for age, sex, and type of cancer treatment. CONCLUSION: Sleep-wake cycle disturbance, inattention, and perceptual disturbances were the most common delirium symptoms in cancer patients. The severity of delirium in patients taking zolpidem was significantly greater.