Non-convulsive status epilepticus (NCS) is rarely encountered and may appear with a psychiatric mask. As clouding of consciousness is the major ictal manifestation, the condition may easily be overlooked in the mentally retarded. We have studied 11 mentally retarded patients with NCS. Since NCS with a focal onset may have a generalized ictal EEG pattern, a classification of NCS solely based on the seizure classification may be misleading. In some patients, it is impossible, both clinically and on the basis of EEG recordings, to distinguish between continuous complex partial seizures and atypical absences. We therefore propose a revised classification of NCS based on the ictal EEG pattern and the epilepsy syndrome diagnosis (I) NCS in generalized epilepsy syndromes, (II) NCS in localization-related epilepsy, (a) with localized EEG features, (b) with generalized EEG features, and (c) with transitional EEG features, and (III) undetermined NCS. Four of our patients were classified as Group I, two as Group IIa, one as Group IIb, one as Group IIc, and three as Group III. Benzodiazepines at small or standard doses may be ineffective in terminating NCS, particularly in the Lennox-Gastaut Syndrome. The identification of trigger factors is essential. Drugs seemed to be the most important precipitants in our patients; in three, NCS was induced by recurrent rectal diazepam over-administration. This complication of rectal diazepam treatment in epilepsy has not been addressed previously.