Background Factors associated with delirium among community-dwelling older adults have been poorly studied. Our aim was to describe the prevalence of predisposing and precipitating factors for delirium among patients admitted for delirium and to assess whether these factors were appropriately recognized at the first patient assessment at hospital. Methods Consecutive community-dwelling individuals admitted to three geriatric acute care units with a confirmed initial diagnosis of delirium were prospectively included. An independent investigator recorded, using a predefined form, any acute medical condition considered by the attending geriatrician to be a precipitating factor, at the first patient assessment and at the end of his stay in acute care. Results A total of 208 patients were included, 80.0% had a pre-existing cognitive or neurological disorder, or both. The most frequent precipitating factor found were infections (49.0% of all patients, mainly lung and urinary tract infections), followed by drugs (30.8%), dehydration (26.4%) and electrolytic disturbances (18.7%, mostly hyponatremia). 91% of patients had a cerebral imagery, but acute neurological conditions were found in only 18.3%. Fewer precipitating factors were found at first than at final assessment (1.4 (95%CI 1.3-1.6) versus 1.9 (95%CI 1.8-2.0) respectively, p<0.001). This difference was significant for all main categories of precipitating factors.