Background: The purpose of this investigation was to evaluate and compare the changes due to extraction and non-extraction treatments after twin-block therapy. Methods: Forty Class II division 1, mandibular retrognathic patients were included in the study. Group I comprised 14 girls and 6 boys who has the indication of mandibular premolar extraction after twin-block therapy. Group II comprised 12 girls and 8 boys treated with twin-block appliance and fixed orthodontic therapy without extraction. Lateral cephalometric radiographs were obtained from the patients at the onset of the treatment (T1), at the end of the use of the twin-block appliance (T2) and at the end of active treatment (T3). The Paired Sample t test and Independent Sample t test was used for statistical analyses. Results: After twin-block therapy, SNB and lower incisor angles increased in both groups while ANB and convexity decreased. In the extraction group, lower lip moved backwards, upper and lower incisors were retracted during the T2-T3 period. In the non-extraction group, only the lower lip vertical length increased. Significant differences were found between the two groups in terms of changes in lower and upper incisor positions/inclinations and lower lip thickness in T2-T3 period (p <0.05). Conclusion: Even though extraction and non-extraction treatments after twin-block make a difference in the final lower and upper incisor positions and inclinations, they do not create a significant difference in terms of their effects on soft tissues.