For 30 years the Royal Air Force has used the EEG as part of the medical screening of candidates for aircrew training. A total of 13,658 males aged 17-25 years have been examined. None had a previous history of significant illness. Sixty-nine (0.5%) of these showed unequivocal epileptiform discharges, 44 (58%) occurring only on photic stimulation. Those with EEG discharges of uncertain significance such as 6 and 14 Hz positive spikes, 6 Hz spike and wave, and non-specific paroxysmal activity were not included. A group of 43 with a follow-up period of 5-29 years have been reviewed. Only one person developed unequivocal epilepsy. If this rate is combined with results from similar studies, the chance of healthy individuals with EEG abnormalities of this type subsequently developing epilepsy is 2-3%.