In a focus of infantile visceral leishmaniasis in Al Agamy (Alexandria), Egypt, adults were found by radioimmune assay to have leishmanial antibodies. This finding was unexpected, and an explanation was sought. Seropositive adults and age- and sex-matched seronegative controls were interviewed to ascertain possible predisposing risk factors. Ten of 16 seropositive individuals were taking corticosteroids compared to one of 22 seronegative individuals (P = 0.0002). Two seropositive individuals (one woman and one man) were taking phenylbutazone, and two women were taking female sex hormones. The association between immunosuppressive medication and leishmanial seropositivity suggests that acquired changes in host population immunocompetence may significantly alter age-specific seroprevalence of leishmanial antibodies.