Purpose To evaluate outcomes and prognostic factors in patients with acute ischemic stroke caused by tandem internal carotid artery/middle cerebral artery occlusion undergoing endovascular treatment. Materials and Methods Characteristics of consecutive patients with tandem occlusion (TO) were extracted from a prospective registry. Collateral vessel quality on pretreatment computed tomographic (CT) angiography was evaluated on a 4-point grading scale, and patients were dichotomized as having poor or good collateral flow. Outcome measures included successful reperfusion according to Thrombolysis In Cerebral Infarction score, good outcome at 3 months defined as a modified Rankin scale score ≤ 2, symptomatic intracranial hemorrhage (ICH; sICH), and mortality. Results A total of 72 patients with TO (mean age, 65.6 y ± 12.8) were treated. Intravenous thrombolysis was performed in 54.1% of patients, and a carotid stent was inserted in 48.6%. Successful reperfusion was achieved in 64% of patients, and a good outcome was achieved in 32%. sICH occurred in 12.5% of patients, and the overall mortality rate was 32%. Univariate analysis demonstrated that good outcome was associated with good collateral flow ( P = .0001), successful reperfusion ( P = .001), and lower rate of any ICH ( P = .02) and sICH ( P = .04). On multivariate analysis, good collateral flow (odds ratio [OR], 0.18; 95% confidence interval [CI], 0.04–0.75; P = .01) and age (OR, 1.08; 95% CI, 1.01–1.15; P = .01) were the only predictors of good outcome. The use of more than one device for thrombectomy was the only predictor of sICH (OR, 10.74; 95% CI, 1.37–84.13; P = .02). Conclusions Endovascular treatment for TO resulted in good outcomes. Collateral flow and age were independent predictors of good clinical outcomes at 3 months.