A longitudinal study was done in a leishmaniasis -endemic region in eastern Sudan during the period November 2001-February 2003 to determine the incidence of failure of sodium stibogluconate treatment. We studied 820 confirmed visceral leishmaniasis patients. All were treated with sodium stibogluconate, 20 mg/kg body weight for at least 28 days. Parasites were isolated from lymph node aspirates from 22 participants identified as relapsed patients. All isolates were typed as Leishmania donovanibased on polymerase chain reaction (PCR) amplification of parasite kDNA. Six parasites showed in vitro resistance to sodium stibogluconate using murine J774 macrophage amastigote testing method. The resistant isolates showed different restriction profiles when the amplified kDNA PCR products were digested with ALU1 restriction enzyme, indicating that resistance was mediated by different parasite clones.