Microfilariae of Onchocerca volvulus were found in the cerebrospinal fluid (CSF) of 5/8 heavily infected onchocerciasis patients. During treatment with diethylcarbamazine citrate 10/11 patients showed increased numbers of 0. volvulus microfilariae in the CSF. Patients with concentrations of 8-31 mf/ml CSF developed severe vertigo, and some other neurological manifestations, during treatment. A hypothesis is put forward to account for this clinical piciture, and its importance in relation to the treatment of onchocerciasis is discussed.