Plasma prolidase activities (PPAs) in cases of gastric cancer. This study was based on a prospective design. 60 patients with gastric cancer (operable cases: 48; inoperable: 12) and 60 healthy controls were included in the study. Prolidase activity was analyzed. The mean PPA levels in the operable, inoperable and control groups were 45.60 ± 4.20, 54.35 ± 4. 9 and 10.92 ± 0.79 U/l, respectively (p < 0.001). PPA decreased significantly after tumor resection. PPA level was significant in differentiating operable cases from inoperable (sensitivity: 91.7%; specificity: 85.2%). PPA was significantly higher in patients with inoperable gastric cancer than in operable cases and the control group. A strong correlation was found between tumor volume and PPA.