The methods used for the assessment of adult Onchocerca volvulus by histology are described. Based on the results of several studies, mainly in Liberia and Burkina Faso, the morphology of the adult filariae in histological sections is represented as far as it is relevant for the evaluation. Especially are described the morphological alterations due to old age of the worms, to chronic hyperreactivity of the human host (sowda) and effects of the macrofilaricidal suramin and of microfilaricidal drugs. Quantitative results are reported on untreated adult O. volvulus from various countries, the changes of the worm population during 12 years of vector control in Burkina Faso and the effects of suramin, diethylcarbamazine, metrifonate, and ivermectin. The data from the histological examinations are compared with those gained from the examination of worms isolated by the collagenase technique in the same studies.