Experimental infection of dogs with Leishmania infantum has yielded heterogeneous clinical, parasitologic, and immunologic results. We studied dogs infected with 10(5) or 10(4) sand fly-derived promastigotes delivered by the intradermal (ID) or intravenous (IV) routes. Total mortality over 1 year post-infection reached 23.8%. The mortality and proportion of sustained polysymptomatic dogs was highest in the IV-10(5) group. The early appearance of polysymptoms was associated with an increased risk of progression to death. Dissemination of the parasite to lymph nodes was faster, and the subsequent infectivity to sand flies higher, in the IV compared with ID-infected dogs. Parasite-specific IgG1 or IgG2 production was similar among the groups, but higher interferon-gamma (IFN-gamma) and interleukin-10 (IL-10) expression was associated with polysymptomatic dogs. On the basis of the data obtained from this study, a sample size analysis using different endpoints for future vaccine trials is described.