Frequent interictal epileptiform discharges (IEDs) may have effects on cognition. We analysed a group of 182 children with different epilepsy syndromes, all of whom had IEDs that did not result in observed seizures, with 24-h ambulatory EEG and cognitive tests. The IED index was estimated, in wakefulness and in sleep, as percentage of time in five categories (0%, = 10-50% and >= 50%). IEDs were defined as spikes or spike-wave complexes, isolated or occurring serially (in runs) without evident clinical signs of a seizure. The IED categories were correlated to cognitive test results and epilepsy characteristics. The group of patients with diurnal IEDs in >= 10% of the EEG record showed impaired central information processing speed, short-term verbal memory and visual-motor integration. This effect was seen independently from other EEG-related and epilepsy-related characteristics, and independently from epilepsy syndrome diagnosis. The impact of the nocturnal IEDs was of less importance; only contributing partially to the slowing of central information processing speed. We conclude that frequent IEDs (in more than 10% of the record) in the awake EEG can impair cognitive performance in children. Whether children with a high diurnal spike frequency and low seizure frequency can benefit from antiepileptic treatment should be examined in controlled trials.