Bcl‐2 homology domain 3 (BH3)‐only protein Bid is posttranslationally cleaved by caspase‐8 into its truncated form (tBid) and couples with stress signals to the mitochondrial cell death pathway. However, the physiological relevance of Bid is not clearly understood. Hepatocyte‐specific knockout (KO) of Bcl‐xL leads to naturally‐occurring apoptosis despite co‐expression of Mcl‐1, which shares a similar anti‐apoptotic function. We generated Bcl‐xL KO, Bcl‐xL/Bid double KO, Bcl‐xL/Bak double KO, Bcl‐xL/Bax double KO, and Bcl‐xL/Bak/Bax triple KO mice and found that hepatocyte apoptosis caused by Bcl‐xL deficiency was completely dependent on Bak and Bax, and surprisingly on Bid. This indicated that, in the absence of Bid, Bcl‐xL is not required for the integrity of differentiated hepatocytes, suggesting a complicated interaction between core Bcl‐2 family proteins and BH3‐only proteins even in a physiological setting. Indeed, a small but significant level of tBid was present in wild‐type liver under physiological conditions. tBid was capable of binding to Bcl‐xL and displacing Bak and Bax from Bcl‐xL, leading to release of cytochrome c from wild‐type mitochondria. Bcl‐xL–deficient mitochondria were more susceptible to tBid‐induced cytochrome c release. Finally, administration of ABT‐737, a pharmacological inhibitor of Bcl‐2/Bcl‐xL, caused Bak/Bax‐dependent liver injury, but this was clearly ameliorated with a Bid KO background. Conclusion: Bid, originally considered to be a sensor for apoptotic stimuli, is constitutively active in healthy liver cells and is involved in the Bak/Bax‐dependent mitochondrial cell death pathway. Healthy liver cells are addicted to a single Bcl‐2–like molecule because of BH3 stresses, and therefore special caution may be required for the use of the Bcl‐2 inhibitor for cancer therapy. (HEPATOLOGY 2009.)