The value of p53 to predict the cytotoxic effect of two commonly used chemotherapy regimens was assessed in patients with advanced breast cancer. Response to a DNA-damaging combination therapy [fluorouracil, epirubicin, cyclophosphamide (FEC] considered to induce p53-dependent apoptosis was compared with a microtubule stabilizing therapy (paclitaxel) expected to be independent of p53 function. The p53 status of the patients' breast tumors was assessed using both immunohistochemistry (IHC) and direct sequencing of the entire p53 gene. p53 findings were correlated with treatment response, and linkage between p53 function and cellular response was assessed by terminal deoxynucleotidyl transferase-mediated nick end labeling assay. In a series of 67 breast tumors, 19% had TP53 gene mutations, 40% had a positive p53 IHC, and 12% had both. In the FEC group, treatment failure was related to both the presence of TP53 gene mutations (P = 0.0029) and a positive IHC (P < 0.0001). Apoptosis was almost exclusively found in tumors having normal p53 in both parameters (P < 0.0001). In the paclitaxel group, treatment response was neither related to apoptosis nor to normal p53. Combination of sequencing and IHC results revealed a significant association between abnormal p53 and response to paclitaxel (P = 0.011). We found TP53 mutations, as well as p53 protein overexpression, to be associated with response to chemotherapy. Whereas clinical response to FEC was found to be dependent on normal p53, the cytotoxicity of paclitaxel was related to defective p53. The efficiency of paclitaxel during mitosis might be supported by lack of G1 arrest due to p53 deficiency. Therefore, patients with p53-deficient tumors may benefit from paclitaxel.