BACKGROUND: A canine vaccine remains a promising approach for effective control of visceral leishmaniasis (VL), given its complex epidemiology in areas where zoonotic VL is prevalent. Leish-Tec(®) is a recombinant vaccine, based on the Leishmania A2 antigen, against canine VL (CVL). It is, since 2014, the single commercial vaccine licensed in Brazil. Here, Leish-Tec(®) efficacy was estimated through a randomized field trial (RFT), in a highly VL endemic area. METHODS: The RFT was conducted from 2008 to 2010 in an endemic area of southeastern Brazil, presenting a CVL seroprevalence of 41.9%. Eight hundred forty-seven seronegative dogs were randomly selected to receive Leish-Tec(®) (n=429) or placebo (n=418). Animals were followed up by clinical, serological, and parasitological exams for 18 months. The CVL incidence in both groups was compared through proportion analysis. RESULTS: A significant reduction in the number of cases of CVL was observed in the vaccine group, as compared with the placebo group, whether efficacy was estimated according to parasitological results (71.4%; 95% CI: 34.9-87.3%; p=0.001; risk ratio=0.287), by adding results of xenodiagnosis and parasitological exams (58.1%; 95% CI: 26.0-76.3%; p=0.002; risk ratio=0.419). Among the animals that converted to a positive anti-A2 serology, efficacy reached 80.8% (95% CI: 37.6-94.1%, p=0.001; risk ratio=0.192). Xenodiagnosis has detected a reduction of 46.6% (p=0.05) in transmission to sand flies from vaccinated animals presenting anti-A2 positive serology. CONCLUSION: The Leish-Tec(®) vaccine proved significantly effective for prophylaxis of CVL, after natural challenge assured by transmission of Leishmania parasites, in a highly endemic area. Noteworthy, this report has unveiled the complexity of performing a RFT for anti-CVL vaccines in Brazil, which may be helpful for designing of future studies.