Purpose <p>The purpose of this study was to evaluate the natural history of radiolucent lines (RLLs) in modern cementless posterior-stabilized (PS) mobile-bearing total knee arthroplasty (TKA) at 3-year follow-up.</p> Methods <p>Our cohort was composed of 45 cementless and 45 cemented TKAs. Patients were retrospectively reviewed after 1:1 matching for age, gender, body mass index, and preoperative UCLA score. All operative procedures were performed by a single surgeon using a cementless or cemented TKA of the same design between 2019 and 2021. The incidence of RLLs was reviewed at 1 week, 6 months, 1 year, 2 years, and 3 years postoperatively. Bone on-growth over the surface of the implants were also reviewed at 3 years postoperatively. Fisher’s exact test, independent Student’s <i>t</i>-test, or Mann-Whitney <i>U</i> test were used for statistical analyses.</p> Results <p>Although the incidence of RLLs was significantly higher in cementless TKAs (33/45 knees; 73%) than that in cemented TKAs (10/45 knees; 22%, <i>P</i> &lt;.01) at 6 months postoperatively, RLLs in cementless TKAs disappeared in 29 of 33 knees (88%) at 3-year postoperatively and those of cemented TKAs had increased up to 17 of 45 knees (38%). Bone on-growth over the surface of the implants was observed in 43 of 45 knees (93%) in the cementless TKAs at 3 years postoperatively.</p> Conclusion <p>This study showed that nonprogressive RLLs in recently introduced cementless PS mobile-bearing TKA designs are expected to disappear over time.</p> Level of evidence <p>Retrospective cohort study, Level III.</p>