OBJECTIVE: Absolute or relative increase in the latency of the major surface positive component is almost invariably found in patients with demyelination optic neuropathy. Using the pattern- reversal method, our study illustrates the significant changes in the latency of the P100 component when refractive errors are introduced to defocus in normal person. METHOD: Four women and ten men aged 20 to 27 years were selected after a thorough ophthalmological assessment. Visual acuity (VA) was 6/6 or better in all subject and none had dyschromatopsia or significant astigmatism. Refractive errors were created by the combined standard lenses. RESULTS: The mean value of P100 latency were as follows: 93.74 +/- 3.30 msec, naked eyes; 98.14 +/- 7.37 msec, the +2/+x 90 lens; 96.50 +/- 3.76 msec, the +1/ +1 x90 lens; 94.55 +/- 4.20 msec, the -1/ -1x90; 96.29 +/- 2.88 msec, the -2/ -2 x90 lens. The P100 latencies showed singnificant standard lens except with -1/ -1 x90 lens. The P100 latency was prolonged according to the progression of refractive error. CONCLUSION: Because a relative or absolute prolongation of P100 latency is often found in cases of suspected multiple sclerosis, and because of their similarity to the findings of our study, we would emphasize that refractive errors should be reduced or eliminated to minimize the false-positive results.