Paclitaxel is a widely used naturally occurring antineoplastic agent that has shown great promise in the treatment of a variety of human solid tumors, particularly for advanced breast and ovarian cancers. Recent studies in our laboratory discovered that glucocorticoids could selectively inhibit paclitaxel-induced apoptosis in a number of human solid tumor cell linesin vitro. Since glucocorticoids (such as dexamethasone) are routinely used as a premedication in the clinical application of paclitaxel to prevent hypersensitivity reactions and other adverse effects, the inhibitory effect of glucocorticoids on paclitaxel-induced apoptosis has raised a clinically relevant question as to whether the pretreatment with glucocorticoids might interfere with the therapeutic efficacy of paclitaxel. In the present study, through development of animal models bearing human breast and ovarian xenograft tumors, we evaluated the potential influence of dexamethasone on antitumor activity of paclitaxelin vivo. The results demonstrated that pretreatment of dexamethasone significantly attenuated therapeutic efficacy of paclitaxel against human breast and ovarian xenograft tumors. The inhibition rate of 20 mg paclitaxel/kg on the growth of breast and ovarian xenograft tumors was around 20–25% less when the animals were pretreated with 1 mg dexamethasone/kg. Further analyses with histological examination and TdT-mediated dUTP nick end labeling assay indicate that pretreatment with dexamethasone clearly interferes with the cytotoxic effects of paclitaxel on both morphological alterations and induction of cell death. Additionally, immunohistochemical staining of proliferation marker Ki-67 indicates that the percentage of proliferating cells in xenograft tumors with pretreatment of dexamethasone is much higher than that in the tumors treated with paclitaxel alone. Put together, the results obtained from the animal experiments show that pretreatment of xenograft tumors with dexamethasone results in significant inhibition of the therapeutic efficacy of paclitaxelin vivo. This finding may have a potential implication on the clinical practice of paclitaxel-based chemotherapy.