A sample of 502 individuals from 94 families from Jacobina, State of Bahia, Brazil, was investigated to determine the causal mechanisms involved in Leishmania chagasi (the causal agent of visceral leishmaniasis in the American hemisphere) infection, as measured by the intradermic reaction to antigens derived from this parasite, using complex segregation analyses. The results showed evidence of a major genetic mechanism acting on infection, with a frequency of a recessive (or additive) susceptibility gene (q) of approximately 0.45. A small multifactorial component (H = 0.29) acting in conjunction with a major recessive gene (q = 0.37) is not ruled out as a concomitant causative factor.