Objectives: Transient postoperative hyperperfusion may be the cause of the temporary neurologic deterioration (TND) frequently observed following superficial temporal artery-middle cerebral artery (STA-MCA) bypass surgery. The purpose of this study is to investigate the possible factors influencing postoperative temporary neurologic deterioration. Methods: The STA-MCA bypass surgery was performed in 13 patients with moyamoya disease and 9 patients with cerebrovascular occlusive disease. The diameter of the STA and MCA for anastomosis was measured and analyzed. Computed tomographic perfusion (CTP) was performed prior to the operation and on the third post-operative day to assess improvements in regional cerebral perfusion. Results: After surgery, five patients (22.7 %) with improvement of perfusion suffered TND such as dysphasia, motor deficit and sensory disturbance after surgery and 11 (50 %) patients with improvement of perfusion did not show temporary neurologic deterioration after surgery. There were six patients (27.3 %) without improvement of perfusion after surgery. The D-STA was 1.48 ± 0.92 mm in patients with TND, which was larger than the D-STA of 1.10 ± 0.14 mm in patients with improvement of perfusion and 1.08 ± 0.26 mm in patients without improvement of perfusion (P < 0.05); The D-MCA after surgery was also wider in patients with TND (D-MCA: 1.20 ± 0.19 mm) compared to both patients with hyperperfusion (D-MCA: 0.93 ± 0.15 mm, p = 0.028) and patients without hyperperfusion (D-MCA: 0.98 ± 0.12 mm, p = 0.012). The TOT was 34.40 ± 7.70 minutes in patients with TND, 32.83 ± 3.43 minutes in patients with improvement in perfusion and 33.45 ± 4.48 minutes in patients without improvement of perfusion; there was no significant difference between the three groups (P >0.05). Conclusions: The D-STA and D-MCA may influence the development of TND after STA-MCA bypass, but the TOT of the recipient arteries was not related to temporary neurologic deterioration.