Thromboembolic disease associated with assisted reproductive techniques is considered to be extremely rare but most serious complication. The reasons for this are thought to hypercoagulable state characteristic of OHSS due to high serum levels of estrogen, hemoconcentration and reduced circulating blood volume, but is still unclear. The risk is increased those with rare hypercoagulable conditions such as antiphospholipid antibody syndrome, protein C deficiency, protein S deficiency, antithrombin III deficiency, and those with a personal or family history of thromboembolic disease. The majority of thrombosis reported were venous site but arterial thrombosis mostly intracerebral was reported 5 cases in Korea so far. We present a case of basilar a. thrombosis at 11 days after hCG injection. The patient developed the right hemiparesis, and recovered after intraarterial thrombolysis and transluminal angioplasty. Protein S activity was decreased and vWF antigen was increased. Decreased protein S activity was also found in previous reported 4 cases, so we suggest screening test for protein S in OHSS patients.