Seven dogs experimentally infected with amastigotes or culture promastigotes of Leishmania infantum MON-1 were observed for a period of up to 38 months. The course of infection was monitored by clinical and parasitological examinations, haematological and serum protein analysis, and by anti-leishmania antibody levels. Two of the three amastigote-inoculated dogs developed a symptomatic infection with haematological and protein alterations, and a strong humoral immune response. The third dog was asymptomatic with no haematological or protein alterations and developed a steady humoral response. Four promastigote-inoculated dogs remained asymptomatic throughout the observation period, with only transient antibody responses to leishmanial antigen, and no haematological or protein alterations. The detection of the parasite in biological material obtained at necropsy showed that dogs with no clinical signs or other manifestations of disease may be infected. This indicates that asymptomatic carriers may be present in the canine population, but not identifiable by the usual serological tests, and suggests that epidemiological surveys based on serology may underestimate the prevalence of canine leishmaniosis and the parasite transmission risk.