Patients (39) (Group A) with ocular onchocerciasis in the Sudan-savanna of North Cameroon were given 4-6 g of suramin and followed in detail over 1-2 yr. Other patients (39) (Group B) received suramin followed 2 wk later by a 6-7 day course of diethylcarbamazine (DEC). A further 18 patients (Group C) received placebo injections and were followed in the same way by the same observers. Suramin caused serious general reactions among the 100 patients who started the course: 1 case of stomatitis, 1 exfoliative dermatitis and several cases of severe prostration, among which 2 ended fatally. These reactions underline the urgency for further studies on the toxicity of suramin, which is without doubt an efficient macro- and micro-filaricidal drug. Changes which occurred in the ocular lesions are described in detail. There was an initial aggravation of punctate and sclerosing keratitis and sometimes a serious aggravation or development of anterior uveitis, corresponding to the peak microfilaricidal effect of the drug. The possibility of a simultaneous adverse effect on the optic disc is discussed. Despite these reactions, which might have been avoided by prior elimination of microfilariae by DEC, the eyes were in general quieter at 3 mo. and thereafter than before treatment. No posterior segment lesion improved after suramin and the majority remained unchanged. The findings at the end of the trial were delineated.