An investigation was conducted to determine whether seasonal variations affected the development of cutaneous leishmaniasis. Data from 499 cases treated between July 1994 and December 1998 were analyzed. The interval between infection and consultation and between treatment and clinical cure varied significantly between cases with an incubation period during the dry season compared with the rainy season (P < 0.001). When the incubation period occurred during the dry season, the standard pentamidine isethionate treatment seemed to be less effective (i.e., the odds ratio for failure was 1.9 [1.1–3.4], P = 0.01). The presence of lymphangitis was more frequent during the dry season (i.e., the odds ratio was 0.26 [0.15–0.45]m P < 0.001). These results suggested that the observed seasonal variations were due to variations in the host/parasite balance. Converging indirect elements that suggest a role for variations in solar ultraviolet radiation are discussed.