Background: The intermittent delta activity in electroencephalographies (EEGs) of patients with focal brain lesions has been reported to be a marker of an epileptogenic focus. This study investigated the concordance between the current source distribution (CSD) of the interictal epileptiform discharges (IEDs) and that of the background delta frequency bands (DFBs) of the scalp EEG. Methods: We collected scalp EEGs of 13 patients with focal epilepsy that contained uniregional IEDs and unilateral delta to theta slow waves. We applied a distributed source model using LORETA$^{(R)}$ to determine the CSD of the peak points of the IEDs and the DFBs of the background activity. Results: The CSDs of the DFBs were ipsilateral to the CSDs of the peak point of the averaged IEDs in ten patients, and bilateral with ipsilateral predominance in three patients. In the cases with an ipsilateral CSD of the DFB, 8 of 10 patients had concordance of the CSD localization between the averaged IED and the DFB. In the cases with bilateral CSD of the DFB, 2 of 3 patients had concordance of the CSD localization between the averaged IED and the DFB. Conclusions: The CSD localization and lateralization appear to be concordant between the IEDs and the DFB of background activity in epileptic patients. Therefore, the CSD of the DFB in EEGs with visually observable slow activities may predict those of IEDs.