We assessed the frequency of retinal toxicity in patients receiving either chloroquine or hydroxychloroquine and factors which may predict patient susceptibility to toxicity. The overall frequency of retinopathy was 6% (7 of 110 patients). Of the 31 patients receiving chloroquine alone, 6 developed toxicity (19%). In contrast, of the 66 patients receiving hydroxychloroquine, none developed retinopathy. Retinopathy was associated with greater age and with greater accumulative doses of chloroquine. Thus, hydroxychloroquine can be used safely with minimal risk of toxicity.