A prospective study was carried out in a general hospital in Reykjavík to evaluate the prevalence of delirium and dementia among 331 patients 70 years and older who were admitted as an emergency to the medical department. Cognitive function was screened with Mental Status Questionnaire (MSQ) and Mini-Mental State Examination (MMSE) and diagnosed according to DSM-III-R for delirium and dementia. Other information obtained included social and demographic factors, drug consumption, the main condition underlying the delirium and outcome. Severe cognitive dysfunction was present in 32% of all acutely admitted patients 70 years and older, which were diagnosed further as delirium 14% and dementia 18%. At follow-up, concurrent dementia was found in 70% of the delirium patients. The main causes for delirium were cardiac failure 27%, stroke 22% and sepsis 16% and the mortality rate was 32% compared with 8% in dementia alone. The prognosis of patients with delirium and dementia depends on detecting these disorders, and the clinical skills of physicians working with acutely ill elderly patients can be improved by relatively simple screening questionnaires such as the MSQ and MMSE.