Multifocal ERG ring ratios provide a sensitive and objective method to detect ocular toxicity in patients taking hydroxychloroquine (Plaquenil). In order to measure ring ratios, the average mfERG amplitude was calculated for each of five concentric rings of a 61-hexagon mfERG. The age-corrected amplitude of the central hexagon (R(1)) and the ratios of R(1) to each of the successive rings (R(1)/R(2), R(1)/R(3), etc.) were then computed. Normative values for ring ratios were established from a population of 67 normal controls. In the study population, a ring was considered abnormal if it was above the 99% confidence limits for the normal population. The technique was evaluated on 131 eyes of 62 patients taking Plaquenil for a variety of conditions including rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome. Patients who had taken Plaquenil for an extended period showed a higher incidence of retinal toxicity, regardless of the condition for which they were taking the drug. Among patients who had taken a cumulative dose of less than 1,250 g, 7 of 67 eyes (10%) showed a characteristic mfERG defect, while in patients with a cumulative dosage of 1,250 g or more, 26 of 64 eyes (41%) showed one of these defects. In at least one patient, the technique was able to detect the early onset of Plaquenil toxicity followed by reversal of the toxic effects after the medication was discontinued. It appears appropriate to recommend that mfERG testing be done on all patients on Plaquenil therapy.