Septic arthritis associated with extra-articular abscess is rare. We report on 2 non-rheumatic patients with septic arthritis of the knee associated with calf abscess. Magnetic resonance imaging showed a distinct leakage pathway from the knee joint in each patient. One was a ruptured popliteal cyst (posteromedial). Another was a pathologic popliteus hiatus (posterolateral). These patients underwent open drainage of the calf abscess via a small incision, followed by arthroscopic debridement of the knee. Careful palpation of the lower leg, followed by magnetic resonance imaging and needle aspiration, is important to exclude a possible extra-articular abscess regardless of the presence of a popliteal cyst.