To evaluate the relationship between collateral circulation and ischemic patterns of ischemic stroke with unilateral severe symptomatic middle cerebral artery stenosis. Methods: We retrospectively investigated 187 patients who suffered of ischemic stroke with unilateral severe symptomatic unilateral middle cerebral artery (MCA) stenosis. They were divided according to status of collateral circulation into the good collateral circulation group (n=79) and poor collateral circulation group (n=108). The neurological functions of the 2 groups were compared, and the ischemic patterns and its relationship with collateral circulations in the 2 groups were analyzed. Results: Compared to the poor collateral circulation group, the good collateral circulation group showed a lower NIH Stroke Scale (NIHSS) score and incidence of early deterioration of neurological function (both P<0.05). The good collateral circulation group presented mostly small subcortical infarctions and the poor collateral circulation group mostly internal border zone infarctions and multiple infarctions. The differences in the ischemic patterns of the 2 groups were statistically significant (P<0.05). Analysis of cerebral perfusion parameters showed that, in the good collateral circulation group, the time to peak (TTP), cerebral blood volume (CBV), and mean transit time (MTT) in the affected side were greater than those in the healthy side (P<0.05). In the poor collateral circulation group, the TTP and cerebral blood flow (CBF) in the affected side were greater than those in the healthy side, and the MTT was lower than that in the healthy side (P<0.05). In the good collateral circulation group, the relative CBV (rCBV) and rMTT of the affected side were higher than those in the poor collateral circulation group, while rTTP and rCBF were lower than those in the poor collateral circulation group (P<0.05). Conclusion: Collateral circulation is helpful in maintaining the cerebral perfusion in ischemic stroke caused by severe symptomatic unilateral MCA stenosis, improving the composition of infarct patterns, reducing the occurrence of early neurologic deterioration, and improving neurological function scores.