Clinically visceral leishmaniasis is suspected in only a fraction of infected persons, as the majority of these may not have clinical manifestations and remain asymptomatic. There is scanty information on diagnosing latent infections and predicting disease in asymptomatic persons. We therefore carried out a study on asymptomatic contacts of patients with visceral leishmaniasis and post-kala-azar dermal leishmaniasis by using methods for detection of antibody to recombinant K39 (rK39) antigen. A total of 240 patients with leishmaniasis and 150 asymptomatic contacts were tested for anti-rK39 immunoglobulin G (IgG) and IgA antibodies. Fifty-five asymptomatic persons were found to be seropositive. These individuals were monitored every 3 months for 1 year. On follow-up, 43.9% of the asymptomatic seropositive contacts developed kala-azar within the first 3 months, and a cumulative total of 69% developed kala-azar within 1 year. The rest remained asymptomatic and self-healed the infection. The sensitivity and specificity of rK39 enzyme-linked immunosorbent assay (ELISA) and dipstick tests were 100%, while an in-house-developed latex agglutination test had 80% sensitivity. The antibody profile showed that the IgG anti-rK39 antibodies reached a titer of up to 10(-6) within 6 months of infection, started declining thereafter, and completely disappeared in 2 to 3 years in successfully treated cases. Significant titers of IgA antibodies were detectable a little earlier than those of IgG antibodies and were undetectable after 6 months. The study showed that mass screening of family members and contacts by using anti-rK39 ELISA could be a highly reliable tool for early diagnosis and to plan prophylactic treatment of latently infected asymptomatic carriers to eradicate kala-azar.