Objective <p>To investigate if changes in body mass index (BMI) result in changes of the mandibular trabecular bone structure.</p> Materials and methods <p>Females (18–35 years at baseline, mean BMI 42,3) were followed from before (<i>n</i> = 117) until two years (<i>n</i> = 66) after obesity treatment (medical or surgical). The mandibular bone trabeculation was classified as sparse, dense, or mixed on intraoral radiographs (Lindh’s index). A digitized method (Jaw-X) assessed the size and intensities of intertrabecular spaces. The main predictor variable was BMI reduction over the period.</p> Results <p>Before treatment, the group with a high BMI (≥ 45) had a significantly denser bone than those with a lower BMI (<i>p</i> = 0.035). Two years after treatment, fewer were classified with sparse bone (Lindh’s index <i>p</i> = 0.001, Jaw-X <i>p</i> = 0.009). The physical activity increased with fewer having a sedentary lifestyle (40% before, 17% after treatment). The association between BMI reduction and the difference in Jaw-X was significant in regression models and not influenced by obesity treatment method but by baseline factors as age, trabecular bone pattern and level of ionized calcium.</p> Conclusions <p>Before obesity treatment, high BMI was associated with dense bone trabeculation in the jaw. The group with sparse bone had decreased at follow-up. The association between BMI reduction and bone trabeculation was influenced by individual and medical factors.</p> Clinical relevance <p>Bone trabeculation in the mandible was maintained during the first years after obesity treatment but new health habits should be encouraged, and patients need to be monitored and followed up further.</p>