The authors describe a surgical procedure to 'fine-tune' the tibial slope during high-tibial osteotomy. Fifteen consecutive patients were treated for medial compartment osteoarthritis of the knee using a temporary unilateral external fixator and accompanying internal fixator composed of two plates (with different sized space). All 15 patients were evaluated by measuring femoro-tibial angles (FTAs) in the frontal plane, and using the proximal tibial anatomical axis (PTAA) and the posterior tibial cortex (PTC) methods to assess tibial slope in the sagittal plane. FTA, PTAA, and PTC angles were measured using: (1) radiographs taken before surgery, (2) fluoroscopic images taken after inserting the first plate during surgery, (3) fluoroscopic images taken after tibial slope restoration using an external fixator system during surgery, and finally using, (4) radiographs taken after surgery. In all patients, preoperative PTAA and PTC angles increased significantly after inserting the first plate posteromedially at osteotomy site. After tibial slope had been accurately restored using the external fixator system, PTAA and PTC angles decreased to the preoperative tibial slope level without changing femorotibial angles in the coronal plane. The authors were able to acquire a consistent and reproducible natural tibial slope using tibial slope "fine tuning" using an external fixator and a stable internal fixator.