Objective: The objective of this study was to assess the prognostic value of a high or immeasurable ankle-brachial index (ABI) at baseline for major amputation and amputation-free survival (AFS) in patients with critical limb ischemia (CLI). Methods: Data from two recent trials in patients with CLI and proven infrapopliteal arterial obstructive disease were pooled. Patients were allocated to the low (<0.7), intermediate (0.7-1.4), or high (>1.4)/immeasurable ABI subgroup. Major amputation and AFS rates were compared. Hazard ratios for major amputation and death were calculated. The net reclassification improvement of incorporating high/immeasurable ABI in the Project of Ex-Vivo vein graft Engineering via Transfection III (PREVENT III) prediction model was derived. Results: There were 146 patients (56.2%) who had a low ABI, 81 patients (31.2%) who had an intermediate ABI, and 33 patients (12.7%) who had a high/immeasurable ABI at baseline. Patients with high/immeasurable ABI showed higher 5-year major amputation (52.1%) and lower 5-year AFS (5.0%) rates than the intermediate (25.5% and 41.6%, respectively) and low ABI patients (23.5% and 46.9%, respectively; both P <.001). This same trend was observed in subgroup analysis of diabetics and nondiabetics. Adjusted hazard ratio of high/immeasurable ABI for major amputation/death risk was 2.93 (P <.001). Adding a high/immeasurable ABI as model factor to the PREVENT III model yielded a net reclassification index of 0.38 (P <.0001). Conclusions: A high/immeasurable ABI in patients with CLI and infrapopliteal arterial obstructive disease is an independent risk factor of major amputation and of poor AFS, in both diabetics and nondiabetics. Incorporating high/immeasurable ABI in the PREVENT III prediction model improves its performance.