The fucose-mannose ligand (FML)-ELISA assay showed a sensitivity of 100% and a specificity of 100% in diagnosis of canine visceral leishmaniasis (CVL) (kala-azar) in sera from naturally infected dogs from São Gonçalo do Amaranto, Rio Grande de Norte, Brazil. The overall prevalence of antibodies to Leishmania in the endemic area was 23% (79 of 343). Seroreactivity detected by a Leishmania chagasi immunofluorescent (IF) assay was much lower (2.9%) and similar to the percentage of dogs with kala-azar symptoms (2.6%). Twenty-one of 21 asymptomatic, FML-seropositive animals died of kala-azar in a period ranging from 0 to 6 months after diagnosis. The predictive value was 100% for the FML-ELISA, 43% for an L. mexicana ELISA, and 24% for the L. mexicana and L. chagasi IF assays, respectively. In experimentally infected dogs, all assays detected seropositivity between 90 and 120 days after infection. Since the current strategy for control of CVL is based on detection and destruction of infected dogs, the highly predictive, sensitive, and specific FML-ELISA represents a useful tool for field control of the disease.