It is widely assumed that the recording of EEG infraslow activity (ISA) requires direct current amplifiers. Yet, it has been shown during the past decade that conventional EEG systems can record activity between 0.01 and 0.1 Hz and that this frequency band contains additional information, especially in regard to seizure onset. To delineate the characteristics of background ISA, 24-hour scalp and intracranial recordings obtained from 5 patients during long-term monitoring were investigated. Scalp recordings had been sampled but intracranial tracings were continuous over periods of up to 3 days. Although scalp recordings were subject to artifact, intermittent genuine ISA increase could occur episodically in the interictal state not only distant from the ictal onset zone but also in the contralateral hemisphere. Intracranial recordings were limited, in this study, to portions of one hemisphere but likewise revealed that distant areas could show ISA increase lasting minutes or hours. This was at times not detectable when only the conventional frequency band was viewed. It is concluded that ISA information is contained in the clinical setting of intensive video-monitoring studies for the detection of the epileptogenic zone and can provide additional information above and beyond what is seen in the conventional frequencies.