A retrospective review of 100 admissions to Denver General Hospital with a diagnosis of acute organic brain syndrome was conducted. A total of 44% of the patients were found to have a chronic organic brain syndrome with a superimposed acute insult which caused decompensation. The other 56% of patients developed acute organic brain syndromes de novo for a variety of reasons. The most common etiologic factors producing decompensation of the chronic OBS were infections (in 23%) and environmental changes (in 17%). The most common etiologic factor causing AOBS de novo was drug-related. In most cases, a toxicologic screen, lumbar puncture, and CT scan of the brain should be a part of the investigation of any patient with AOBS.