Symptom assessment in the palliative care unit must consider the reduced physical and mental status of the patients. Standardized instruments are often not completed by patients with cognitive impairment. We tried to combine minimal burden for patients and staff with sufficient information content in a Minimal Documentation System (MIDOS) for pain and symptom assessment in palliative care patients. From January to July 1998, 108 patients (123 consecutive admissions) with a mean age of 63 years (range 32-87 years) were admitted to the palliative care unit. Pain was reported as the reason for admission in 70% of the patients, and 71% were treated with opioids. Using a cut-off point of 20/21, 35% of the patients were impaired in the Mini Mental State Examination (MMSE). The number of missing values in the Brief Pain Inventory (BPI) and the quality-of-life questionnaire SF-12 correlated highly with each other and with the MMSE sum score, but not with the summary scores of BPI or SF-12. Only 31 patients completed the SF-12 quality-of-life questionnaire. Age was not correlated to MMSE scores, and neither were opioid doses for 26 patients with slow-release oral morphine or for 20 patients with transdermal fentanyl. Only a minority of patients was able to use the numerical scale for symptoms other than pain, though most patients were able to score symptom intensity on the verbal categorical scale. Pain and symptom assessments were performed by the physician for 17% of the patients at admission, and for 16% of the follow-up controls because self-assessment was not possible. In this study, cognitive impairment prevented symptom assessment with longer and more complicated instruments such as the SF-12 in a large number of the patients admitted to the palliative care unit. Assessment instruments for patients with advanced disease must provide simple categorical scales and the possibility of being administered by interview.