To study electroretinographic (ERG) response to light flashes in patients with choroidal melanoma and to define possible factors involved in the modification of both a- and b-wave.ISCEV standard flash-ERG was recorded from both affected and control eyes on 24 patients before surgical operation (local excision or enucleation). The choroidal melanomatous mass ranged from 33 to 2880mm(3). Tissues from both melanomatous retina-choroid complex and areas far from the melanoma (unaffected) were taken from 13 enucleated eyes to measure the level of FGF2 mRNA, utilizing the technique of semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Tissues from 10 normal eyes were used as control. The majority of patients showed a marked a- and b-wave attenuation in the affected eye with respect to the fellow eye. In 10 retinal specimens, the expression of FGF2 mRNA showed an increase in retinal regions far from the melanoma compared to control eyes. Many patients present an increase in the expression of FGF2 mRNA in the unaffected part of the retina and a clear attenuation in both a- and b-ERG components. The size of melanoma does not predict the amount of reduction in the ERG response, at least for sizes less than 1000mm(3). We suggest that the melanoma triggers a process leading to an up-regulation of FGF2 in the human eye and this up-regulation might be responsible for the ERG attenuation.