Oral mucositis (OM) is the non-hematological toxicity with the highest prevalence and morbidity in anticancer treatment. This study evaluated the use of low-level laser for the prevention of chemotherapy-induced OM by comparing 101 cycles with prophylactic irradiation using a gallium aluminum arsenide (GaAlAs) laser diode (λ=660 nm; P=40 mW, dose of 4 J/cm²) and 121 cycles with no irradiation. The conditions associated with oral health, chemotherapy cycles, neutropenia patterns, infectious complications and nutritional status were evaluated. OM occurred in 41.9% of the cycles. The probability of developing OM in the final cycles (7 to 10) was 7.34 times higher than in the initial cycles (1 to 6); 4.19 times higher in febrile neutropenia than in physiological neutropenia; 2.08 times higher when a therapeutic antimicrobial drug was used; and 2.12 times higher when gingivitis was present. After finding similarity between the groups with respect to the variables studied, it was concluded that the application of prophylactic laser did not reduce the frequency of chemotherapy-induced OM but was effective in preventing severe forms of the disease, reducing the occurrence of OM grades III and IV from 22% to 7% with no adverse effects, which justifies its routine use.