The isometric muscle strength of the muscles around 49 medial compartmental osteoarthritic knees of 32 women was measured quantitatively using a Musculater GT-50 (osteoarthritis group). The patients pushed up a pad containing an electrical sensor and values were compared with those obtained from the nonaffected knees of subjects (control group). The mean age of the patients in the osteoarthritis group was 62.3 years (range, 43-78 years). The control group consisted of 13 women (13 knees) with a mean age of 60.5 years (range, 49-73 years). In the osteoarthritis group, the medial joint space was preserved in 17 knees (Grade A), 23 knees had narrowing of the joint space (Grade 2), and nine knees showed joint space obliteration (Grade 3). Quadriceps strength was significantly weaker in Grade 2 knees than in Grade 0 knees; however, there was no significant difference in quadriceps to hamstrings ratio between each grade. The hip adductors to hamstrings ratio was significantly greater in Grade 2 knees than in Grade 0 knees, and was greater in Grade 3 knees than in Grade 0, Grade 1, and Grade 2 knees. The findings suggested that in patients with medial compartmental osteoarthritis of the knee, the hip adductor muscles may become strengthened to decrease varus decrease varus deformity of the limb.