A single 10-14 day course of diethylcarbamazine (DEC) was given under betamethazone cover to 18 patients (Group A) with ocular onchocerciasis, and the effects were studied in detail over 1-2 years. A comparison was made with the findings in 21 patients (Group B) who received a similar course to be followed by a weekly suppressive 100-200 mg dose of DEC. A further 18 patients (Group C) served as controls. The initial DEC course provoked severe general reactions, but ocular complications could be controlled by betamethazone. The weekly suppressive dose was not acceptable to the majority of patients, and only one young patient with severe iritis who improved during the initial course took the weekly tablet voluntarily over 2 years. The initial DEC course reduced the numbers of microfilariae in the eye and was of temporary benefit to lesions of the anterior segment, but it did not affect lesions of the posterior segment. A possible adverse effect on the optic disc is discussed. Any beneficial long term effect was almost confined to Group B, and to lesions of the anterior segment. The best to hope for in lesions of the posterior segment was arrest of further development. The findings at the end of the trial were as follows: No. of lesions (see article).