Visceral leishmaniasis (VL) is a zoonosis in most regions where it occurs. Dogs are the most important reservoir of the disease and are mainly responsible for the persistence of VL in the Paleartic and Neotropical regions. Canine leishmaniasis (CaL) is a viscerocutaneous, chronic infection with a worse prognosis than human disease. We now know that, as in man, there are some cases of asymptomatic infection. Former studies indicated that dog cutaneous parasitism becomes infectious to the insect vector in later periods of the disease, but recent studies performed by xenodiagnosis have shown that it is possible that transmission might occur earlier. The infected animal reacts with a great production of antibodies and depression of cellular immunity. Antibodies are not protective and resistance is related with active cellular immunity. The presence of Th 1 response in asymptomatic animals, sometimes without humoral response, means that the prevalence of CaL, found in epidemiological surveys by searching for antibodies, may be underestimated.