The oral mucositis is the most frequent acute oral complication resulting from antineoplastic treatment and may worsen the clinical condition of the patient and interfere with his/her quality of life. This study aimed to comparatively evaluate, from a clinical point of view, the effect of Laser Therapy λ660 nm (wavelength of the red Laser) and λ830 nm (wavelength of the infrared Laser), at extra oral points, in remis sion of severity of oral mucositis and pain associated with it in pediatric oncological patients undergoing che motherapy with the anticancer drug methotrexate, noting which of the two wavelength is the most appropri ate to this new technique. The sample consisted of 13 patients placed at random in each group and subjected to sessions of Low Level Laser Therapy, at pre determined extra oral points for five consecutive days, starting at the beginning of the observation of mucositis injuries. It became possible to note that from the group of patients in the group of Laser λ830 nm (n = 6; 46.15%), four (n = 4; 66.67%) of these patients had remission of injuries to grade 0 (WHO), and as for pain, five patients (n = 5; 83.33%) showed no painful symptoms for mucositis injuries. In the Laser λ660 nm group (n = 7; 53.85%), only two patients (n = 2; 28.57%) achieved a regression of lesions to grade 0 (WHO), while four patients (n = 4; 57.14%) had no pain. So, the extra oral application of Laser Therapy was effective in treating injuries of oral mucositis in the patients treated; and Laser Therapy in the infrared spectrum (λ830 nm) was more effective in the treatment of oral mucositis inju ries compared to the red spectrum (λ660 nm), which can be explained by the greater power of penetration of infrared rays, acting in a more expressive way in deeper places.