Contrast sensitivity function (CSF) was measured in 16 patients (14-60 yr.) with epilepsy to investigate adverse effect of antiepileptic drugs on the central nervous system. Eight patients were treated with phenytoin, while 8 were given polytherapy (phenytoin in combination with phenobarbital, carbamazepine or valproic acid). Thirty-one normal controls (19-59 yr.) were also subjected to this study. Vertical sinusoidal gratings with various spatial frequencies (0.5-20.0 c/deg) were presented on a video monitor. Contrast sensitivity (reciprocal of threshold contrast) was determined at each spatial frequency. CSF of normal subjects showed an inverted U-shaped function against the spatial frequencies with a peak at 6 c/deg (medium size pattern). There was no significant difference in CSF values between normal controls and patients with epilepsy. However, 3 patients with polytherapy showed the significant reduction of contrast threshold. Since these patients did not complain of visual disturbance with normal visual acuity, CSF abnormality was considered as having subclinical visual dysfunction. These results suggest that CSF is useful for evaluating the adverse effect of antiepileptic drugs on the visual recognition, and that polytherapy is responsible for CSF abnormality. Therefore, monotherapy should be scheduled from the onset of therapy.