Delirium is common and serious acute syndrome among older people precipitated by multiple external factors such as acute illnesses, trauma, surgery, and drugs. The aim of this study was to find possible stressors and causative triggers for acute delirium and compare patients with or without dementia in this respect. 193 delirious patients from two separate delirium studies including settings of nursing homes and geriatric wards were thoroughly assessed for precipitating factors of delirium. Patients with and without dementia were compared for their clinical status, symptoms and signs, prognosis, and the profile of precipitating factors of delirium. The patients with dementia (n=98) and without dementia (n=95) did not differ in their demographic factors, mean number of drugs, or their psychiatric symptoms. The patients with dementia had higher number of comorbidities, poorer cognition, and they were more often restrained than those without dementia. The mean number of precipitators for delirium was 2.6 among those without dementia and 2.0 among those with dementia (p=0.0019). Infections, metabolic conditions, trauma, and surgery were more common precipitating factors for delirium in those without than those with dementia. There was no difference in mortality between the groups. Most patients had multiple precipitating factors for delirium irrespective of prior dementia. Those with dementia and decreased cognitive reserves needed lower number of etiologies to develop delirium. The profile of causative agents differed among patients with and without dementia. •Delirium has very often multiple etiological factors.•Patients without prior dementia require more stress triggers than those with dementia.•The etiologies differ among patients with and without dementia.•Both groups had very poor prognosis and there was no difference between the groups in mortality.