The objective of this study was to clarify the clinical features of long-time survivors with unresectable pancreatic cancer treated by gemcitabine(GEM)alone and to predict survival time by carbohydrate antigen(CA)19-9, enhanced computed tomography(CT), and 18F-fluoro-2-deoxy-D-glucose positron emission tomography(FDG-PET) in monitoring the response to chemotherapy. Twenty-one patients with unresectable pancreatic cancer were enrolled in this study. All patients were evaluated by serum CA19-9 level, tumor size of CT, maximum standardized uptake value (SUVmax)with FDG-PET and other factors before chemotherapy(GEM alone at a dose of 1,000 mg/m(2) weekly x 3 followed by 1 week of rest), and they received chemotherapy until obviously progressive disease. Serum CA19-9, tumor size of CT and SUVmax with PET were measured after three courses of chemotherapy in ten patients. We compared these three modalities in terms of two points: Which is the best modality to predict survival time ? Which is the best monitoring modality to evaluate the efficacy of chemotherapy on unresectable pancreatic cancer ? A significant difference in survival time was not found between high level group and low level group of serum CA19-9 level and SUVmax with FDG-PET and also longest length of tumor by enhanced CT. In ten patients we evaluated the response rate of each parameter CA19-9(IU/mL), CT(longest length of tumor), and SUVmax with FDG-PET. We defined the response rate(pretreatment level of CA19-9 or longest length of tumor or SUVmax-after 3 courses chemotherapy level of CA19-9 or longest length of tumor or SUVmax/pretreatment level of CA19-9 or longest length of tumor or SUVmax). Response rate of CA19-9 was significantly correlated with survival time(r=0.633, p=0.0481). However, the response rate of SUVmax with FDG-PET had no significant correlation with survival time(r=0.019, p=0.9630). In the present study, the response rate of CA19-9 is the best monitoring modality to evaluate the efficacy of chemotherapy.