Introduction: "Non-aromatic Very rich in Steranes" (NAVS) naphtalan is a natural mineral oil, produced by various methods of separation, with a preserved sterane structure which has healing properties. The purpose of this study was to evaluate the effectiveness of NAVS in the treatment of oral lichen planus (OLP) and recurrent aphthous stomatitis (RAS). Material and Methods: A randomized, double-blind trial was designed. Fifty seven patients (30 OLP, 27 RAS) were randomized into two groups to receive NAVS or 0,05%-betamethasone dipropionate in adhesive paste 3x daily for 28 days. The severity of the OLP lesions (REU) was objectively scored. The number and the diameter of RAS lesions were assessed on days 0, 3 and 5. The intensity of the pain/discomfort was determined using a visual analogue scale (VAS) and OHIP-14-questionnaire before and after the therapy. Results: Greater REU reduction was observed in NAVS group (NAVS 67,21%, betamethasone 66,30%), although it was not statistically significant (p=0,4421). No statistically significant differences in the reduction of the OHIP-14 between NAVS (74,38%) and betamethasone group (67,55%) were observed (p=0,8191). VAS reduction for NAVS was 96,29% and for betamethasone 93,66% (p=0,3722). There were no statistically significant differences between the two groups in reduction of number (day 3. p=0,48; day 5. p=0,75) and lesion diameter (day 3. p=0,40; day 5. p=0,83) in RAS patients. VAS and OHIP-14 reduction was observed in both groups 8 days after the completion of therapy, but the difference was not statistically significant (RAS p=00,59; OHIP-14 p=0,69). Conclusion: NAVS has successfully reduced the clinical signs and symptoms of OLP. It has reduced the number, diameter and symptoms in patients with RAS.