This paper introduces a safe arthroscopic technique passing through the posterior septum from the posteromedial cotnpartment to the posterolateral compartment, or vice versa, without damage to the PCL, capsule, and/or neurovascular structures. The posterior septum is triangular, bounded by the PCL anteriorly, the posterior portion of the femoral I.ntercondylar notch superiorly, the posterior capsule posteriorly, and devicles the posterior compartment of the knee joint into the posteromedial and posterolateral compartments. The middle genicular vessel pierces the posterior capsule and runs along the superior border of the posteriorseptum. There are no dangerous important structures at the central portion of the triangle of the posterior septum. This arthroscopic posterior portal technique is devided 4 steps. The first step is to make a posteromedial portal under direct arthroscopic visualization. The second step is to make a posterolateral portal. The third step is to make a hole at the posterior septum and to examine throposterolatera] compartment. The fourth step is to examine the posteromedial compartment through the posterior septum. This technique provides complete visualization of the posterior compartment of the knee joint, including the posterior aspect of the both femoral condyles, the posterior hom of both menisci, the PCL, the meniscofemoral ligament (Wrisberg s ligament), the posterior aspect of the pop liteal tendon, the femoral attachment of the ACL, the posterior septum and the posterior capsule. We have used this technique from October, 1994 to June, 1996 in 58 knees; arthroscopic total synovectomy in 43, removal of the loose bodies located behind the PCL in 8, PCL re!construction in 4, meniscal repair with all inside suture technique in 3. In conclusion, this procedure is very helpful in performing arthroscopic total synovectomy in chronic, acute, hemophilic, and crystal induced synovitis, in removing encapsulated loose bodies located behind the PCL, and in suturing peripheral tears of the posterior hom of the medial rneniscus with an all inside suture technique.