The aim of this study was to evaluate the relationship between occlusal characteristics and oral health-related quality of life in adults who underwent orthodontic or orthodontic-surgical treatment. The study group consisted of 51 adult patients (35 women, 16 men) with severe malocclusion and considerable functional disorders. Thirty-six of the patients underwent combined orthodontic-surgical treatment, while 15 underwent orthodontic treatment. Data were collected before and after treatment. Mean follow-up period was 5.0 years (range 2.2-6.7 years). Occlusal characteristics were measured from dental casts by using Peer Assessment Rating (PAR) index. A self-completed Oral Health Impact Profile (OHIP-14) was used to measure oral impacts. The changes in PAR and OHIP-14 were measured, and correlation between PAR and OHIP tested before and after treatment and in the changes during the follow-up. Statistical significance was evaluated with the paired samples t-test and Mann-Whitney U-test, and the correlation between PAR and OHIP scores assessed using Pearson's and Spearman's correlation coefficient. The occlusion was significantly improved in all subjects, mean PAR reduction being 78.1 per cent. The prevalences of oral impacts at threshold `fairly often' or `very often' before and after treatment were 70.6 per cent and 9.8 per cent, respectively (P < 0.001). The PAR and OHIP scores correlated after treatment but not before treatment or in the changes. The treatment of severe malocclusion reduced the reported oral impacts to the level of general population and significantly improved oral health-related quality of life.