BACKGROUND: No study exists that directly measures the distances between posterior portals and the popliteal artery under arthroscopic conditions. PURPOSE: To define the anatomic relationship between the neural structures and standard posterior arthroscopic portals and between the popliteal artery and posterior as well as transseptal portals in different knee positions. STUDY DESIGN: Descriptive laboratory study. METHODS: Seventeen fresh-frozen cadaveric knees were used. The posterolateral, posteromedial, and transseptal portals were established at 90° of knee flexion. The popliteal artery was revealed using a shaver placed through the posteromedial portal. The distance from those portals to the popliteal artery was measured using a precision caliper. After the measurements were made, each specimen was dissected. The distance from a needle, blade, and cannula in each portal site to the adjacent neural structures was successively measured at 30°, 90°, and 120° of knee flexion. RESULTS: The mean distance (in millimeters) from the posterolateral, posteromedial, and transseptal portals to the popliteal artery was significantly smaller at 30° (32.1 ± 4.6, 36.5 ± 4.9, and 9.0 ± 3.9, respectively) than at 90° (40.7 ± 5.1, 41.0 ± 3.8, and 18.0 ± 3.8, respectively) and 120° (38.4 ± 4.8, 38.7 ± 6.0, and 21.0 ± 4.0, respectively) of knee flexion. The mean distance from the posterolateral portal to the common peroneal nerve at 90° of flexion (needle, 26.6 ± 9.5; blade, 24.7 ± 6.9; cannula, 22.1 ± 6.9) was significantly greater than the distance at 30° (needle, 23.4 ± 6.5; blade, 21.4 ± 6.4; cannula, 18.4 ± 6.3) and 120° (needle, 21.8 ± 6.6; blade, 19.1 ± 6.3; cannula, 17.4 ± 6.7) of knee flexion. The mean distance between the posteromedial portal and the inferior infrapatellar branch of the saphenous nerve at 30° (needle, 18.6 ± 4.3; blade, 15.5 ± 3.3; cannula, 13.7 ± 5.8) of flexion was smaller than at 90° (needle, 20.1 ± 6.1; blade, 16.5 ± 5.3; cannula, 14.3 ± 4.4) and 120° (needle, 21.1 ± 3.6; blade, 17.7 ± 4.9; cannula, 15.1 ± 5.9) of flexion, but there was no statistical significance. The mean distance from the posteromedial portal to the sartorial branch of the saphenous nerve at 30° (needle, 22.8 ± 6.1; blade, 19.8 ± 5.3; cannula, 17.7 ± 6.2) of flexion was significantly smaller than that at 90° (needle, 29.7 ± 3.6; blade, 26.3 ± 6.3; cannula, 23.1 ± 4.7) and 120° (needle, 31.5 ± 3.9; blade, 28.9 ± 4.1; cannula, 25.4 ± 5.1) of flexion. Conclusion/ CLINICAL RELEVANCE: The position of 90° of knee flexion is safe to establish posterior and transseptal arthroscopic portals. The position of 120° of knee flexion is practically safe to establish posteromedial and transseptal portals, but it is unsafe to create a posterolateral portal because the risk of damaging the common peroneal nerve is high. The position of 30° of knee flexion is not recommended to establish posterior arthroscopic portals.