So far, studies of Leishmania persistence in mice have used injections of parasites administered either intravenously in the tail vein or subcutaneously in the footpad. These routes poorly reflect the natural conditions when the sandfly delivers metacyclic promastigotes intradermally. In this study B10D2 and BALB/c mice were inoculated within the ear dermis with 10 4 Leishmania major metacyclic promastigotes. The parasite load was monitored by quantitative PCR in different tissues from the dermal inoculation site to distant tissues. The two sites of multiplication and persistence of parasites were the site of L. major inoculation and the draining lymph node (DLN), with a different pattern in the two mouse inbred lines. These two organs were the only sites harboring parasites 12 months postinoculation, with the DLN of BALB/c mice harboring around 10 7 parasites, a stable load from months 3 to 12. In these two sites, 8 and 12 months after inoculation, interleukin 4 (IL-4), gamma interferon, and inducible nitric oxide synthase transcripts parallel the parasite load while IL-10 transcript levels remain high. In addition, at early time points until month 3, parasite DNA was also detected in distant tissues such as the contralateral noninoculated ear or the tail skin, indicating that blood was at least transiently disseminating the parasites. In contrast, L. major DNA in liver, spleen, and femoral bone marrow remained sporadic in mice of both lines. This study is discussed within the framework of Leishmania transmission from the vertebrate host to the sandfly vector, a complex process still poorly understood.Leishmaniasis currently affects some 12 million individuals in 88 countries, and at least 350 million people are exposed to the risk of the Leishmania parasite inoculation (see the World Health Organization information at http://www.who.int/emc /diseases/leish/leis.html). It is well established that in Leishmania transmission areas, individuals may harbor the parasites at very low levels, without developing symptoms. These individuals, considered asymptomatic carriers, are likely to transmit the parasite to the hematophagous sandfly vector or, as shown recently, via blood transfusion (16). Moreover, Leishmania spp. may persist in cured hosts after drug therapy (for a review, see reference 1) and be reactivated after immunosuppression, for instance, in human immunodeficiency virus-infected people (3). The capacity of the parasite to establish persistent infection as a means to achieve its transmission and hence maintenance of its life cycle is a process common to many parasites (9, 10). Little is known on the mechanisms underlying persistence of Leishmania parasites in their vertebrate host and transmission to the sandflies, pool-feeder hematophagous insects expected to recover transmissible parasites from the dermis. In a recent study using C57BL/6 mice, described as mice resistant to L. major, Stenger et al. (21) showed that a small number of parasites may persist in the regional lymph node, the spleen, and in s...