A major surface glycoprotein of Leishmania parasites, gp63, is highly conserved among species and is expressed in both infective and intracellular stages. Although much is known about its role in entry and survival in host macrophages, patient antibody responses to this glycoprotein have not been well characterized. The prevalence of anti-gp63 antibody in sera of leishmaniasis patients was evaluated by ELISA. Sera from most acute visceral leishmaniasis patients (84%) from Brazil and Sudan had notably high levels of antibody to recombinant (r) gp63. Sera from other forms of leishmaniasis and from other diseases did not contain significantly elevated levels of anti-gp63 antibody. These results indicate that rgp63 might be a useful constituent of a defined serologic test for visceral leishmaniasis.