Non-extraction orthodontic treatment in borderline cases is reported, according to some authors, to lead to upper and lower incisor inclination changes affecting patients' soft tissue profiles. Here, we aim to determine the correlation, in non-extraction Class I malocclusion cases, between the upper and lower incisor inclination changes toward the nasolabial angle and the mentolabial angle before and after orthodontic treatment. (We took before and after lateral cephalometric radiographs on 26 patients with non-extraction Class I malocclusion orthodontic treatment.) We measured the I-SN, IMP, nasolabial, and mentolabial angles. For statistical analysis, we used the non-parametric Wilcoxon test and the Spearmen correlation test. There was no significant difference between the I-SN and nasolabial angles, but a significant difference between the IMP and mentolabial angles, before and after treatment. The correlation test showed very weak negative and positive correlations between the change in the I-SN angle and the change in the nasolabial angle, and the change in the IMP angle and the change in the mentolabial angle, respectively. The decrease in the I-SN angle was followed by an increase in the nasolabial angle, even though the correlation was very weak. The increase in the IMP angle was followed by an increase in the mentolabial angle, which was also very weakly correlated. © Published under licence by IOP Publishing Ltd.