The Merci retrieval system is a useful modality for the recanalization of acute cerebral artery occlusion. However, it remains unclear whether the tortuosity of the middle cerebral artery (MCA) plays a role in successful recanalization. In this study, we investigated the association between the shape of the horizontal MCA segment (M1) and successful recanalization using the Merci retrieval system with or without adjunctive treatments. Twenty-three patients with M1 occlusion underwent thrombectomy using the Merci retrieval system with or without adjunctive treatments between July 2010 and July 2012. The anteroposterior view of final angiograms was used to measure the M1 curve angles. M1 with a curve angle measuring < 100° was defined as arch-type M1, whereas that with a curve angle measuring ≥ 100° was defined as straight-type M1. Angiographic findings were evaluated on the basis of the thrombolysis in cerebral infarction grade; grade 2B or 3 corresponds to successful recanalization. Eight patients had arch-type M1 and 15 patients had straight-type M1. Successful recanalization was achieved in 2 patients (25%) with arch-type M1 and 12 patients (80%) with straight-type M1 (p = 0.023). The mean M1 curve angle was significantly greater in the 14 patients in whom successful recanalization was achieved than in the 9 patients in whom it was not achieved (129 ± 21° vs. 93 ± 29°, p = 0.002). Arch-type M1 was an independent predictive factor of unsuccessful recanalization (odds ratio, 0.045; 95% confidence interval, 0.03-0.696). A tortuous M1 was associated with unsuccessful recanalization by the Merci retrieval system, even when adjunctive treatments were used.