Methods: There will be a Clinical Trial Randomized, double-blind study to evaluate the effectiveness of using low power laser in the prevention of oral mucositis in patients with hematological cancer, undergoing chemotherapy for bone marrow transplantation. Participate in the study, patients who are in the condition mentioned above accepting participate and have aged over 18. Will be excluded from the study patients who have autoimmune disease, which present sensitivity to laser or who have already started treatment for oral mucositis prior to this study.The study variables are:-independent variable: whether or not the red laser of low power-dependent variables: oral mucositis, degree of mucositis, chemotherapy regimen, type of blood cancer. For determining the association between the independent variable and the dependent will be used chi-square tests of association (Pearson) and Fisher's exact test, if necessary. Will calculate the risk ratio (RR) as a measure of relative risk, with the confidence interval at 95% (IC95%).Also be calculated NNT (number needed to treat to obtain benefit) and NNH (number needed to harm getting). Will be adopted a significance level of 5%. The project was approved by the Ethics and Research iMIP. All patients will be appropriately informed about the objectives of the project and will only be included if they voluntarily agree to participate by signing the consent form.