Under the pathological conditions, the VEP may show changes in amplitude, latency, or waveform in one or more of its components. The major advantage of the pattern reversal VEP over the flash VEP lies in smaller variability in the waveform and latency of its components in the healthy population. The flash VEP is, however, particularly useful to infants and newborns. The variation in the waveform of the flash VEP was evaluated, and Type V was abnormal waveform. Subsequently, patients with various CNS diseases were examined. Acute hemiplegic patients showed high amplitude or increased latency patterns. In 45 percent of the West syndrome cases, VEP showed abnormal responses. The patients with asphyxia, respiratory distress syndrome and mental retardation showed increased latency. Latency was decreased in hypoglycemia and hypocalcemia of newborns, and it was increased or decreased in autism and epilepsy. In our study, it was concluded that the rhythmic after-discharge is a true response. This assumption is supported by the following observations: (1) the after-discharge of VEP appeared before the occurrence of the alpha-wave in EEG; (2) the frequency of the after-discharge was generally higher than that in EEG; and (3) the frequency of the after-discharge did not change between the conditions of the resting state and hyperventilation. There was no after-discharge in VEP in patients with a history of encephalitis even when their IQs were normal, and the latency was increased in pattern reversal VEP.