A survival analysis was performed on data from an endemic area of Bolivia where two populations, natives and highland migrants, were living, to investigate risk factors for onset of cutaneous leishmaniasis (CL) and its mucosal form (MCL). In a first data set (703 subjects with 242 CL patients), significant risk factors for CL were gender, native/migrant status, activity, and home-forest distance. The instantaneous risk of CL increased until adolescence in both populations, and rapidly decreased thereafter. This risk was 3-10 times higher in migrants than in natives until 20 years of age, and became similar thereafter. Environmental and behavioral factors did not seem sufficient to explain this contrast between the two populations, and this evolution with age may suggest differences in the mechanisms involved in the development of individual protection during childhood. In a second data set (446 CL patients with 34 mucosal forms) the native/migrant status was the main factor associated with the onset of mucosal form.