Objective <p>This study aimed to compare the effect of bimaxillary first premolar extractions versus non-extraction orthodontic treatment on the vertical skeletal dimension and dentoalveolar structures in Class I malocclusion.</p> Materials and methods <p>A prospective clinical trial was conducted involving 50 adult patients with Class I malocclusion, hyperdivergent facial pattern, and moderate crowding (as low as 4.6 to as high as 7.1 mm) in both arches. The sample was divided into two groups: The extraction group, consisting of 25 patients who underwent extraction of bimaxillary first premolars, and the non-extraction group, composed of 25 patients treated without extraction. The vertical skeletal dimensions and dentoalveolar changes in the three orthogonal planes were assessed on cone beam computed tomography (CBCT) images before and after treatment. Intra- and inter-group comparisons were performed using paired and an independent t-test, respectively.</p> Results <p>In the extraction group, the lower anterior facial height (LAFH) significantly increased by 1.35 ± 1.73 mm (P = 0.00), while the mandibular plane angle (SN-MP angle) showed a non-significant decrease. In the non-extraction group, the LAFH significantly increased by 0.8 ± 1.8 mm (P = 0.03), while the SN-MP angle showed a statistically significant decrease of 0.7 ± 0.75 degrees (P = 0.00). No significant difference was found between the two studied groups in all measurements of the skeletal vertical skeletal dimension including LAFH, posterior facial height (PFH), SN-MP angle, and Frankfort mandibular plane angle (FMA).</p> Conclusions <p>The vertical skeletal dimension showed no significant difference between extraction and non-extraction orthodontic treatments. The influence of first premolar extractions on the vertical skeletal dimension is often overestimated.</p> Clinical relevance <p>The extraction or non-extraction decision in orthodontics is attributed to several factors, in which one of these factors is the impact of treatment on facial divergence of the patient. This study investigates the effect of both treatments and demonstrated the limitations to reduce the vertical skeletal dimension by first premolar extraction. Hence, clinicians need to control the facial divergence by means other than the extraction strategy alone.</p> Trial registration <p>The study protocol was registered at ClinicalTrials.gov with an identification number: NCT04151875.</p>