BACKGROUND The value of multifocal electroretinography (mfERG) in the detection of localized retinal dysfunction in vascular branch disease at the posterior pole of the eye is investigated, and mfERG is compared with visual field defects. METHODS 10 eyes of 7 patients were tested, of which 8 eyes had a branch retinal vein occlusion and 2 eyes a branch artery occlusion. First order mfERG's were performed with the VERIS (TM) system, using 61 and 103 hexagons, and visual fields with the OCTOPUS M1 macular program. The number of hexagons with a response density loss (nV/deg (2)) of more than -2.0 standard deviations or of more than - 1.5 StDev, respectively, found in the normal population was compared with the number of disturbed test locations or total sensitivity loss (dB) of M1. RESULTS Local perimetric defects of various extent correlated well concerning localization and sensitivity loss with mfERG response density loss (nV/deg (2)). The best correlation was the total loss of M1 (dB) versus the number of hexagons out of 103, if the criterion of - 1.5 StDev was applied (p </= 0.001). In most cases, a diffuse but less severe response density loss of mfERG was also found at retinal places outside visual field defects and corresponding visible fundus changes. CONCLUSIONS Local visual field defects in retinal vascular branch occlusion correlate well with mfERG if suitable criteria are applied.