This paper reports the amount of medial and lateral knee joint opening in the general population. Knee joint lateral and medial opening at 20 degrees knee flexion was quantified on manual varus and valgus stress test, respectively, with custom made device. One hundred men and women between the ages of 20-60 years were evaluated for their joint openings. Patients with previous knee surgeries or chronic knee pains were excluded from the study. Measurements were done twice by two different orthopedic surgeons who were blinded from each other. The mean age of the study group was 39 years (range 20-60 years) and 43 years for women (range 20-60 years) and 34 years for men (range 20-60). The mean lateral and medial knee joint space opening was 7.0 degrees (range 3-9 degrees ) and 4.1 degrees (range 2-7 degrees ), respectively, in the overall population; in the male population, it was 6.7 degrees (range 3-9 degrees ) and 3.9 degrees (range 2-7 degrees ), respectively, and in the female population, it was 7.2 degrees (range 3-9 degrees ) and 4.3 degrees (range 3-7 degrees ), respectively. Conversion to displacement in millimetres, the overall mean lateral and medial joint displacement was 9.3 mm (range 5.1-13.6 mm) and 4.8 mm (range 3.5-10.7 mm), respectively; for males, it was 9.1 (range 5.1-11.9 mm) and 4.6 mm (range 3.5-7.9 mm), respectively, for females, it was 9.8 mm (range 7.2-13.6 mm) and 4.9 mm (range 3.7-10.7 mm), respectively. The prevalence of the overall population that exceeds 6 mm or more lateral joint space opening was 91% (male 90% and female 92%) and medial joint space was 8% (male 4% and female 12%). Statistically, significant differences were seen between medial and lateral opening in male, female and the overall population (p<0.001). The female population exhibited wider opening in both medial and lateral joint opening than the male population (p<0.05). The interexaminer reliability showed no significant difference (p>0.05). More than 91% of the Korean population showed wide lateral joint space opening in the bilateral knee. However, none complained of functional instability or symptoms except for non-pathologic laxity detected by the physician. We strongly recommend bilateral comparison of the knee that has wide lateral joint opening.