Familial clustering of filarial infection was investigated through random house-to-house surveys of 643 individuals in Leogane, Haiti, an area with endemic Bancroftian filariasis. Children of infected mothers were 2.4 to 2.9 times more likely to be infected than were those of amicrofilaraemic mothers. Filarial-specific cellular responsiveness in amicrofilaraemic children born to infected mothers was lower than that in amicrofilaraemic children born to amicrofilaraemic mothers. No effect of paternal infection status was seen. The findings show that maternal infection is a risk factor for filarial infection in children and is associated with altered parasite-specific immune reactivity.