We applied subjective and psychophysical methods and pattern visual-evoked potential (VEP) P100peak latency as an objective index to measure spatial modulation transfer function (MTF) in normal subjects and patients with early primary open-angle glaucoma (POAG) in an effort to detect impairment of the spatial MTF of vision. To estimate spatial MTF objectively, the peak latency of the P100 component of pattern VEP was investigated. The mean contrast sensitivity in normal subjects derived from the pattern VEP peak latency was found to be maximum at the check size of 14.4 minutes of arc (i.e., 2.08 cycles per degree of spatial frequency), and the objective spatial MTF curve coincided well with that of the subjective one at all spatial frequencies examined. A statistically significant positive correlation was found between the objective (VEP) contrast threshold and the subjective one. Deterioration of the objective contrast sensitivity derived from the VEP peak latency was found to be much prominent that of the subjective one in patients with early glaucoma.