The objective of this study was to examine the association between preoperative meniscal extrusion of patients undergoing partial medial meniscectomy with clinical outcomes and progression of osteoarthritis and to determine the extent of meniscal extrusion associated with unsatisfactory clinical outcomes and progression of osteoarthritis. Ninety-five patients who underwent partial medial meniscectomy with a minimum follow-up of five years were retrospectively reviewed. Preoperative meniscal extrusion was evaluated with MRI. Patients were assessed preoperatively and postoperatively with Lysholm and IKDC subjective scores for clinical outcomes and with IKDC radiographic scale for osteoarthritis. An ANOVA was used to analyze the variations in meniscal extrusion and the clinical and radiological outcomes. A regression analysis was performed to identify factors that affect preoperative medial meniscus extrusion and that influence results after partial meniscectomy. An optimal cutoff value for meniscal extrusion associated with unsatisfactory clinical outcomes and progression of osteoarthritis was established. Significance was set at P<.05. The mean ± SD preoperative and postoperative Lysholm scores were 59.6 ± 15.5 vs. 83.8 ± 13.1 (P < .001) and the mean preoperative and postoperative IKDC subjective scores were 59.4 ± 16.8 vs. 82.0 ± 15.8 (P < .001). Meniscal extrusion greater than 2.2 mm (sensitivity, 84%; specificity, 81%) and 2.8 mm (sensitivity, 73%; specificity, 85%) was associated with unsatisfactory (poor/fair) Lysholm and IKDC subjective scores, respectively. The progression of osteoarthritis, characterized as a change of at least one category on the IKDC radiographic scale, occurred when meniscal extrusion was greater than 2.2 mm (sensitivity, 63%; specificity, 75%). Patients with higher BMI had significantly greater meniscal extrusion that patients with normal BMI (P < .001). The medial meniscus was more extruded in patients with horizontal and root tears. In conclusion, patients with preoperative meniscal extrusion of 2.2 mm or greater had unsatisfactory clinical outcomes and progression of osteoarthritis after partial medial meniscectomy at a minimum of five years follow-up. Higher BMI and horizontal and root tears were associated with greater preoperative meniscal extrusion.