We have investigated the clinical effects of low power laser therapy (LPLT) on the treatment of conditioning-induced oral mucositis (OM) in patients submitted to hematopoietic stem cell transplantation (HSCT). The evaluation of OM was done using the Oral Mucositis Assessment Scale (OMAS) and World Health Organization (WHO) scale. In the context of a randomized placebo-controlled trial with 38 patients for the evaluation of preventive LPLT, eleven individuals were submitted to allogeneic (AL) HSCT and developed oral mucositis grade 4 (WHO) or a total area of OM of 12 cm (OMAS) and due to that were treated with LPLT with the purpose of symptom relief. The irradiation used was a diode InGaAlP, emitting light at 660 nm, 50 mW and 8 J/cm² measured at the end of fiber optic with 0.196cm² of section area during the treatment. The tip of the laser device touched the oral mucosa and patients recovered on average 6 days (3-12 days) from the beginning of the laser application. Our results have indicated that the use of LPLT in HSCT patients is a powerful instrument in the treatment of overt OM and is now a standard procedure in this group of patients in our hospital.