Purpose. In knee osteoarthritis, the degree of pain varies despite similar imaging findings. If there were quantitative findings related to the pain of knee osteoarthritis, it could be used for diagnosis or screening. The medial meniscal extrusion was investigated as a candidate quantitative finding related to the pain of knee osteoarthritis. Methods. Seventy-six knees of 38 patients (mean age, 73 years) who received intra-articular injections of hyaluronic acid into unilateral knees at the time of diagnosis of knee arthritis were investigated. Cartilage thickness of the femoral medial condyle and medial meniscal extrusion of bilateral knees were measured by ultrasonography. Thirty-eight knees that had hyaluronic acid injections were compared with 38 other side knees from the same patients as the control group. Results. The average cartilage thicknesses of the knees with pain that received intra-articular injections and the knees without pain that received no injections were 1.02 and 1.05 mm, respectively (P = 0.6394). On the other hand, the average medial meniscal extrusions of the knees with and without pain were 7.58 and 5.88 mm, respectively (P = 0.0005); pain was associated with greater medial meniscal extrusions. Conclusion. Medial meniscal extrusion is a quantitative finding related to the pain of knee osteoarthritis.