In Santiago del Estero, an area endemic for Chagas' disease in northwestern Argentina, household dogs were vaccinated with live-attenuated Trypanosoma cruzi, and the prospective incidence of natural infection by this parasite was assessed during a two-year followup period. Vaccinated dogs received 107 attenuated, TCC strain T. cruzi epimastigotes and were given booster vaccinations two and 14 months later. The number of animals that could be evaluated in vaccinated versus control groups was 73 and 75 after one year and 49 and 40 after two years, respectively. Parasitologic evaluation by xenodiagnosis indicated that vaccination had reduced natural T. cruzi infection from 26.7% to 12.3% after one year (P = 0.015). The preventive effect of vaccination after the second year was less significant in spite of the booster vaccinations. Inclusion of indirect hemagglutination data for the diagnosis of infection slightly increased the number of infected dogs without affecting the evidence for protection in the first year. Serologic, parasitologic, and isoenzyme studies indicated that protection was mediated by an attenuated, self-cured infection. In 15 dogs in which the vaccination failed to completely prevent natural infection, immunization nevertheless impaired their ability to infect the natural insect vectors of the disease in humans.