Oral mucositis is a painful condition that occurs in 80% of patients who undergo haematopoietic stem cell transplantation (HSCT). Our objective was to determine the impact of mucositis on quality of life (QoL) of patients subjected to HSCT treated with low-level laser therapy (LLLT). Patients were evaluated: (1) on the first day of treatment; (2) 5 days after autologous or 8 days after allogeneic transplantation; (3) once bone marrow had integrated; and (4) 30 days after discharge. Clinical evaluation was performed using the World Health Organization criteria; oral health QoL was measured using the Oral Health Impact Profile (OHIP-14); and mucositis symptoms with the Patient-Reported Oral Mucositis Symptom (PROMS) scale. The higher the score, the lower the patient's QoL. The OHIP-14 responses showed that at D + 5/D + 8, all domains had the highest scores, while at times 1 and 4, the scores were lower. In the PROMS scale, all domains scored worst at time 2, and the differences between the scores at the four times were statistically significant. The study has shown that QoL improves over time in patients undergoing LLLT therapy for mucositis prevention.