Iatrogenic lesions (chemical or mechanical) constitute a specific cause of oral mucositis (OM). In the treatment of oral mucositis (OM) low-level laser therapy (LLLT), is used because of its analgesic, anti-inflammatory and biostimulating effect. The article presents a literature review concerning LLLT application in dental physiotherapy (including oncological patients) and a case study of a female patient (42-years-old) suffering from an extensive iatrogenic oral mucositis being the consequence of endodontic treatment. The following parameters of low-level laser therapy were used: wavelength 830 nm, power output 400 mW, IIIB, dosage 20 J/cm², continuous fluency, 200 J; 8:20 min; 5 applications. Prior to each LLLT and one week after the first application (6 times in total) an evaluation was carried out using an original Oral Cavity Pain Scale and Laitinen Scale, as well as photographic record. Already after the first, yet particularly visibly, after the second LLLT application a decrease in pain level both using Oral Cavity Pain Scale (respectively by 20 pts/% and 68 pts/%) as well as Laitinen Scale (respectively by 5 pts and 10 pts) was demonstrated in comparison with the primary condition. After 7 days from the first LLLT application the patient felt only slight discomfort during eating (1 pt/% in Oral Cavity Pain Scale). The results of presented treatment and the data obtained from literature encourage the usage of LLLT in OM.