This paper reviews epidemiological data to see if there are sex differentials in prevalence, density and clinical pathology due to lymphatic filariasis. Of 53 studies from Africa, South East Asia, the Indian Subcontinent and The Americas, 43 showed a lower mean prevalence of infection in females than in males. Prevalence is consistently lower in women of reproductive age and this is statistically significant in 16 of 32 studies classified by age and sex. Density of infection is also lower in the reproductive age but may be higher in children and in older women. Clinical disease is also lower in women and pathology has a later age of onset and rise to peak prevalence than in males. The paper assesses the evidence that lower rates of infection and clinical pathology are due to less exposure of females to infective vectors. It seems unlikely that exposure alone could account for these differences which are observed for both bancroftian and brugian filariasis, irrespective of periodicity. Several investigators have suggested that females have increased resistance to infection and this is supported by serological studies showing high antibody positivity to adult worm antigens in females. The review concludes that the association with the reproductive years suggests a pregnancy-associated mechanism. This has important implications for maternal-fetal interactions and maternal filarial infection may influence the development of immunity in children.