BACKGROUND: The duration of electroencephalography (EEG) recordings varies widely among laboratories. Although several recommendations had been published, there are no previous studies directly addressing this. AIMS OF THE STUDY: To assess the effect of the recording duration on detection of EEG abnormalities in a tertiary referral centre for epilepsy. METHODS: We have reviewed 1005 EEG recordings and determined the shortest recording duration necessary to identify interictal EEG abnormalities. RESULTS: Standard, awake recordings shorter than 20 min yielded a significantly lower incidence of abnormal findings as compared to longer recordings. Although there was an increase in the diagnostic yield from 30 to 180 min recording duration, this failed to reach the level of significance. For sleep recordings, there was no significant increase in the diagnostic yield beyond 30 min. CONCLUSIONS: Our results provide evidence for recommending at least 20 min recording duration for standard awake EEGs and 30 min for sleep EEG recordings. As data were derived from patients referred to our epilepsy centre, the results are only valid for epilepsy-related indications.