Purpose: The study was conducted to determine whether low-intensity laser therapy (LLLT) can reduce the duration of chemotherapy- and/or radiotherapy-induced oral mucositis (OM). Materials and Methods: A placebo-controlled randomized study was carried out using LLLT or control (placebo treatment). Patients older than 18 years of age treated with chemo- and/or radiotherapy between October 2005 and May 2006 were eligible as soon as they developed OM. Patients received intervention for 5 days. The laser group was treated with GaAlAs laser, wavelength: 830 nm (infrared), power: 100 mW, dose: 4 J/cm2. The control group received a placebo treatment. The grade of OM was clinically assessed according to the Common Toxicity Criteria Scale of the National Cancer Institute. Thirty-four patients developed OM and were available for analysis; 22 (65%) patients had a diagnosis of solid tumors and 12 (35%) leukemia or lymphoma. The mean age was 41 (± 20) years. Eighteen patients were randomized in the laser group and 16 patients in the control group. Results: Once OM was diagnosed, the patients had daily OM grading assessments before laser or placebo application, and thereafter until complete healing of lesions. On day 7 after OM diagnosis, 32% of patients presented OM in the laser group and 94% of patients in the placebo group (p = 0.001). In the laser group, the mean of OM duration was 6.8 ± 2.2 days, and in the placebo group 11.5 ± 3.5 days (p < 0.001). Conclusion: Our study has shown evidence that laser therapy in addition to oral care can decrease the duration of chemotherapy- and radiotherapy-induced oral mucositis. These results should encourage clinicians to use this technique to improve the quality of life of cancer patients during oncology treatment.