The evidence for male/female differences in prevalence, density of infection and clinical disease due to onchocerciasis is reviewed and related to what is known about differential exposure of females to infective vectors. Sex differentials are most marked in savanna areas of high transmission, and in these areas, worm burdens are lower from early childhood in females--as are ocular lesions. In forest areas, sex differences are less marked and ocular lesions are similar in men and women. Sex differences are most evident under conditions of high transmission and it is suggested that females are more resistant to infection than males. There is little substantive evidence that onchocerciasis is less frequent in females on the basis of exposure but controlled exposure and immunological studies, analysed by age and sex, are needed to confirm this. Little is known about onchocerciasis in pregnancy but increased resistance could influence the risk of transmission of infection from mother to child in highly endemic areas. Onchocerciasis in pregnancy is also likely to affect immune response to tetanus toxoid vaccination in mothers and birthweight of children. The disease therefore represents an important public health problem for women and their offspring.