Focal electroretnnograms (ERGs) were obtained from 35 normal eyes and 48 eyes with known macular disease using the MaculoScope stimulator-ophthalmoscope instrument. Response amplitude and phase were combined to permit interpretation of the ERG responses as two-dimensional vectors. This vector interpretation avoids statistical anomalies and distortions inherent in the use of amplitude and phase response parameters. Furthermore, the vector interpretation permits the use of the Hotelling T2 statistic for the assessment of the significance of the differences between ERG responses, providing greater statistical power than simultaneous comparison of amplitude and phase data separately. The distribution of normal responses yielded limiting normal values of 135 nV for the amplitude and −95° for the phase lag of the focal ERG as recorded by the MaculoScope instrument. Significant abnormalities of the focal ERG are illustrated for cases of cilioretinal artery occlusion, Stargardt's disease, idiopathic macular hole, diabetic macular edema, and idiopathic macular dysfunction with normal fundus. This study demonstrates the potential value of the focal ERG in the demonstration of macular disease undetectable by conventional electroretinography.