PURPOSE: Argininosuccinate synthetase 1 (ASS1) silencing in carcinomas and sarcomas leads to a dependence on extracellular arginine for survival. Arginine deprivation therapies, such as PEGylated arginine deiminase (ADI-PEG20), have shown limited effectiveness, which may be due to underlying mechanisms that inhibit apoptosis. EXPERIMENTAL DESIGN: The effects of ADI-PEG20 on cell-cycle regulation, apoptosis, and BCL-XL-mediated survival pathways in ASS1-deficient cancer cells were determined. The mechanism of cell death protection was determined by assessing caspase and PARP cleavage, CDK2 activity, MCL1 expression, and the interactions among BCL-XL, BAX, and BAK. In vitro synergy was determined, and in vivo efficacy was modeled. RESULTS: Treatment with ADI-PEG20 led to reduced CDK2 activity and inhibited cell-cycle progression but did not induce significant cell death. BCL-XL was found to bind to BAX and BAK, preventing the initiation of apoptosis despite arginine starvation. Inhibition of BCL-XL allowed proapoptotic BAX and BAK to initiate the intrinsic apoptosis pathway, leading to increased cell death. This was found to be synergistic in vitro and efficacious in combination in vivo. CONCLUSIONS: The study identifies BCL-XL as a key factor limiting the efficacy of arginine starvation therapies. Combining BCL-XL inhibitors with arginine deprivation strategies may overcome this resistance and enhance therapeutic outcomes. These findings provide a strong preclinical rationale for testing this combination approach in phase 1 clinical trials for ASS1-deficient cancers.