Thirty one eyes with established glaucoma, 61 high risk ocular hypertensive (OHTs) eyes, 66 medium risk OHT eyes, 58 low risk OHT eyes, and 47 control eyes have been followed for up to 2 years by clinical examination and pattern electroretinography (PERG). The study was 'masked' so electrophysiological and clinical data were kept separate. Criteria have been devised which enable PERG measurements to distinguish all established glaucomatous eyes from all normal controls; these criteria demonstrate abnormalities in some OHT eyes, particularly those at high risk. The PERG abnormality is greatest in eyes with established glaucoma in which the intraocular pressure has been lowered by treatment. The PERG becomes smaller as the degree of clinical abnormality increases. Test-retest variability of the PERG is sufficiently low to ensure that most of those first described as abnormal continue to be so.