Objective: The purpose of this retrospective study was to comparativelyassess the severity of apical root resorption in patients treated with secondpremolar extraction and non-extraction treatment protocols. A further aimwas to evaluate the effect of treatment duration on the degree of apical rootresorption.　　 Methods: The sample consisted of 596 teeth in 50 patients (41 females, 9males) aged 12-22 years, with different malocclusions. The non-extractiongroup included 20 patients, with an initial mean age of 15.10± 3.04 yearsand the extraction group comprised 30 patients treated with secondpremolar extractions, with an initial mean age of 15.07± 3.08 years.Chi-square and two-sample t-tests were performed to ascertain similarity ofthe 2 groups. The groups were well matched in relation to initial age, sex,malocclusion types, overjet and overbite. Apical root resorption in all toothgroups, except maxillary first premolar, maxillary and mandibular secondpremolars and second molars, was evaluated from pre- and post treatmentpanoramic radiographs. The intergroup differences were analyzed with theStudents t-test and the correlation between the degree of root resorptionand treatment time was investigated with the Pearson correlationcoefficient.　　 Results: The second premolar extraction group showed significantlygreater apical root resorption than the non-extraction group for themaxillary and mandibular central incisors, lateral incisors and 1st molars(P< 0.05). Maxillary and mandibular canines of the extraction group didnot show any statistically significant apical root resorption.Mandibular lst premolars ofthe extraction group demonstrated sigruficantlygreater root resorption than the corresponding teeth of the non-extractiongroup (P-O.Ol). Negative values indicating root elongation was seen insome ofthe tooth groups.　　 Even though duration of treatment was longer in the extraction group, itwas not found to be correlated with the amount ofroot resorption.Conclusion: Extraction can significantly influence the degree of apical rootresorption as a result ofincreased tooth movement and apical displacement.Caution should, therefore, be exercised with patients who have undergonepremolar extractions as part of their comprehensive orthodontic treatmentplan.