PURPOSE: The aim of this study was to assess the size and extrusion of the meniscus in young and non-traumatic knees, especially in Korean males. MATERIALS AND METHODS: The participants (n=91 knees, 77 patients) were consecutive patients observed at an orthopedic outpatient clinic who had another problem unrelated to the knee joint. The patients were excluded from the study if they had a history of trauma and pain on the knee joint. The patients received a magnetic resonance imaging (MRI) scan on the knee joint. The size, extrusion, and extrusion ratio of the meniscus on an MRI scan were evaluated. RESULTS: The mean age of the participants was 21.6±1.1 years (range, 19–24 years). The mean size of the medial meniscus on the coronal plane was 9.0±1.1 mm (range, 7.1–11.9 mm). The extent of extrusion on the coronal plane was 2.0±1.0 mm (range, 0–4.4 mm). The extrusion ratio (extruded size/total size×100) of the medial meniscus on the coronal plane was 22.7%±11.5% (range, 0%–53.7%). The extent of extrusion on the sagittal plane was 1.6±1.1 mm (range, 0–4.0 mm). The extrusion ratio (extruded size/total×100) of the medial meniscus on the sagittal plane was 18.0%±11.9% (range, 0%–40.7%). The incidence of medial meniscal extrusion on the coronal and sagittal plane are 92.3% and 80.2%, respectively. The mean size of lateral meniscus on the coronal plane was 11.1±3.4 mm (range, 7.3–22.9 mm). The extent of extrusion on the sagittal plane was 0.5±0.7 mm (range, 0–2.5 mm). The extrusion ratio of the lateral meniscus on the sagittal plane was 4.0%±6.7% (range, 0%–26.3%). The incidence of lateral meniscal extrusion on the coronal plane was 34.1%. No extrusion of the lateral meniscus was observed on the sagittal plane. CONCLUSION: In young and non-traumatic knees, the extrusion of meniscus was common, especially medial meniscus than lateral meniscus.