Oral mucositis is one of the complications arising from pre bone marrow transplant conditioning, which can substantially change the patient's quality of life. The purpose of this randomized double blind study was to compare the effects of low intensity laser radiation in the prevention of oral mucositis in patients undergoing bone marrow transplants. Seventy patients at the Seattle Cancer Care Alliance in the U.S.A. were approved by the local ethics committee and gave their informed consent to take part in the study. The 70 patients were divided into three groups (group 1 - laser 650nm; group 2 - laser 780nm and group 3 placebo). The therapy or placebo treatment began on the first day of the conditioning and continued through to two days following the bone marrow transplant. Mucositis was measured according to the oral mucositis rate and the pain assessment rate (VAS). We were thus able to conclude that the diode 650nm laser indeed decreased the severity of oral mucositis as well as the degree the pain when used as a preventative therapy in patients undergoing bone marrow transplants. In this study, low intensity laser therapy was regarded as safe and did not present any side effects.