PURPOSE: Nonconvulsive status epilepticus (NCSE) is clinically difficult to diagnose, especially in old patients without epilepsy, and requires electroencephalography (EEG) for diagnosis. Its incidence among elderly patients with confusion of unknown origin (CUO) remains undetermined. METHODS: A 1-year prospective study was conducted in patients aged 60 years or older, for whom EEG was requested because of confusion considered to be of unknown origin after initial clinical, biologic, and imaging investigations. Diagnosis criteria included a validated clinical assessment scale to confirm confusion. RESULTS: Of 44 patients with confirmed CUO, 7 presented with de novo NCSE. NCSE population had a mean age of 76 years (range, 60-97 years). No statistically significant differences were found between NCSE patients and others for age, drugs, presence of myoclonia, eyelid myoclonia, tachycardia, or agitation. In contrast, an acute onset (<24 h), gender (100% female among NCSE patients), and lack of clinical response to simple commands were significantly associated with NCSE. No differences between the two groups were evidenced for mortality and morbidity (length of hospitalization, social outcome, and so on). DISCUSSION: Almost 16% of patients aged 60 or older with confusion of unknown origin had NCSE, according to this first prospective study. Female patients with rapid onset (<24 h) of symptoms and lack of response to simple commands were at a higher risk of presenting with NCSE.