An epidemiological study of Ascaris lumbricoides infections was carried out in primary school children aged 5-16 yr from Ile-Ife, Nigeria. Intensity of infection was assessed directly by means of counting worms passed during a 48-hr period after chemotherapy. Reinfection patterns of A. lumbricoides were assessed at 2 6-mo intervals and statistical evidence of predisposition to infection status was obtained. An investigation of 3 groups of children who were judged to be predisposed not to be infected, to be lightly infected, and to be heavily infected was undertaken. Assignment to the groups was based upon the mean worm burden plus 1 SD above the mean, measured at 2 6-mo intervals. The distribution of class I human leucocyte antigens among the 3 groups of children was described. None of the children who were predisposed to remain uninfected was found to possess the A30/31 antigens in contrast to those children who remained infected.