Introduction: Proliferative activity of periodontal ligament (PDL) cells decrease with age in an animal model. Furthermore, human PDL is less cellular in adults than in children. However, there is no quantitative human PDL assessment related to the effect of age on PDL cell proliferation under different durations of orthodontic load. Aim of Study: To determine the differences in proliferative activity of PDL cells on the root surface of orthodontic loading teeth between younger and older groups in different loading periods. Materials and Methods: Fifty-eight patients, requiring premolar extraction for orthodontic treatment, were divided into two groups: younger group (age15-20) and older group (age>20). For loading, 0.016-inch improved super-elastic nickel-titanium alloy wires were used. One of their maxillary first premolars was loaded for one, two, four, six, or eight weeks, whereas the contralateral premolar was not loaded as a control. Premolar extractions were performed, and the extracted premolars were fixed with 10% buffered formalin and stained with toluidine blue. Digitised images were taken under a stereomicroscope, and the percentage of stained PDL was analysed. Differences in the percentage of stained PDL between loaded and unloaded teeth for the different durations of load were analysed using the independent t-test. P-values less than 0.05 were considered statistically significant. Results: The percentage of PDL cell proliferation was significantly greater in the younger group than in the older group after loading for short durations (one to two weeks) (p0.05) but not for long durations (four to eight weeks). In the older group, we found loading for long durations significantly increased the percentage of PDL cell proliferation than did short duration loading (p0.05). Conclusion: In terms of proliferative rate, PDL cells of younger patients respond to orthodontic force faster than older patients. Schlagwörter: Proliferative activity, PDL cell proliferation, orthodontic loading