OBJECTIVE Excessive varus and valgus stress forces during arthroscopy might exceed minimal compressive strength of cancellous bone. In extreme cases, this could lead to post-arthroscopic osteonecrosis. It was our purpose to measure the valgus and varus stress forces during arthroscopy and draw conclusions on the development of osteonecrosis. METHODS On 24 consecutive patients undergoing arthroscopy, the maximum varus and valgus stress forces (N) were measured in vivo using a strain gauge mounted to a leg holder. The forces (N) and contact stresses (kPa) on the femoral condyles were calculated based on the measured acting lateral force at the femur fixation based on the lever principle. RESULTS The maximum contact stress during varus on the medial condyle was significantly lower in patients with intact meniscus (mean ± standard error of the mean: 243 ± 29 kPa) than in patients with meniscus-deficient knees (520 ± 61 kPa; P < 0.01). A similar finding was obtained for the maximum contact stress during valgus on the lateral condyle: 630 ± 72 kPa in patients with intact meniscus compared to 2,173 ± 159 kPa in patients with meniscus-deficient knees (P < 0.01). In 19 patients (79%), the maximum contact stress was higher during valgus than during varus. The maximum contact stress on the lateral condyle during valgus was significantly higher for more experienced surgeons (P = 0.01). CONCLUSIONS The maximum contact stresses in knees with intact menisci did not exceed the critical threshold of the compressive strength in cancellous bone. However, the maximum contact stresses in meniscus-deficient knees were frequently higher than the threshold. However, these stresses were much lower than those during daily activities and therefore unlikely to lead to post-arthroscopic osteonecrosis. METHODS Diagnostic study, Level II.