Pentavalent antimonials have been the standard agents in the treatment of cutaneous leishmaniasis in Brazil since the late forties.To date, there is no clear assessment on the emergence of resistance to pentavalent antimonials in Leishmania and the role of those parasites in treatment failures.Until recently, glucantime was the most widely used first line anti-leishmanial drug in Brazil when a sodium stibogluconate of Chinese origin, another pentavalent antimonial, was put on the market.The purpose of this study was to assess the involvement of resistant Leishmania parasites in the failure to cure leishmaniasis patients.Two groups of patients with cutaneous leishmaniasis were administered standard treatments with either one or the other drug.The two groups were then compared in relation to the cure rate of patients and the corresponding in vitro sensitivity of the parasite to the two drugs.Overall, the cure rate was similar for patients treated with either glucantime or sodium stibogluconate.Out of a total of 27 patients, two relapsed after being treated with sodium stibogluconate and four relapsed after a treatment with glucantime.To assess the role of the intrinsic sensitivity of the parasite to the two drugs, kill curves in vitro (IC50) were performed on those Leishmania brasiliensis showing differences in vivo, patients who relapsed versus patients who cured (the control group).Present data shows low IC50 values for all control parasites tested: 4 with glucantime and 3 with sodium stibogluconate.Two out of three parasites from patients who relapsed after a treatment with glucantime showed elevated IC50 values as did the one parasite evaluated from a patient resistant to a sodium stibogluconate treatment.More parasites are being tested in order to establish if this difference is statistically significant.