The expression of p-glycoprotein (p-gp) was evaluated in pre- and post-chemotherapy states after the administration of adriamycin-based chemotherapy in 24 gastric cancer patients. Among them, group A was composed of twelve patients who relapsed after surgery plus adjuvant chemotherapy and group B was composed of another twelve patients who received neoadjuvant chemotherapy plus surgery. Pre-chemotherapy p-gp was evaluated in 18 out of 24 patients (6 patients had no pre-chemotherapy paraffin blocks) and post-chemotherapy p-gp was evaluated from all 24 patients. Pre- and post-chemotherapy p-gp was expressed in 5 of 18 patients (27.8%), and 9 of 24 patients (37.5%), respectively, with immunohistochemical stain using monoclonal antibody JSB-1. No differences of disease-free survivals were observed in Group A based on post-chemotherapy p-gp expression from relapsed lesions. In Group B, there was a higher relapse rate (p = 0.04) and a lower one-year disease-free survival rate (p = 0.04) in post-chemotherapy p-gp positive patients when adjuvant treatment was done with the same regimen as neoadjuvant chemotherapy. In all patients studied, post-chemotherapy p-gp expression correlated with a higher systemic recurrence (p = 0.04). These data suggest that p-gp can be induced by an adriamycin-based chemotherapy in gastric cancer. Thus, we suggest that the prognosis of gastric cancer may be poor if a multidrug resistance (MDR)-related regimen is used in the presence of p-gp after neoadjuvant chemotherapy with an adriamycin-based regimen, even if the initial response is good.