To investigate early changes of retinal function in myopic eyes, patients with various degrees of myopia underwent multifocal electroretinography (multifocal ERG).Thirty young patients (mean age, 26.1 years) were divided into three groups according to the degree of refractive error: emmetropia/low, medium, and high myopia. All groups were matched by sex and age. All subjects had good corrected visual acuities of better than 1.0 and normal color vision. No other ophthalmoscopic abnormalities except for tessellated fundus existed. Three grouping programs (all-traces, rings, and quadrants) were used, and all-traces grouping waveforms, six-ring grouping waveforms, and four-quadrant grouping waveforms were determined from 103 local responses. We studied the differences of amplitudes and latencies of the waves among the myopic groups and their relation to refractive error and axial length.Amplitudes were reduced and latencies were delayed as the refractive errors increased. The amplitudes in the peripheral areas were more reduced in the myopic groups.The reduced amplitude and delayed latency of each wave of multifocal ERG in myopia resulted primarily from cone function loss, which may occur even when the only noted retinal change is tessellated fundus.