We reviewed electroencephalographic (EEG) records of 626 epileptic pat
ients for the frequency of epileptiform patterns ( EP). The purpose of
this study was to determine the factors determining the occurrence of
EP. Epileptiform pattern was detected in 46% of patients studied. The
occurrence of EP was associated with age, seizure type, longer durati
on of epilepsy, and proximity of EEG examination to last attack. We fo
und no association between seizure frequency, medication status at EEG
examination and the occurrence of EP. The yield of diagnostically use
ful findings in the interictal EEG would be increased if the test is p
erformed as close as possible to the last seizure.