Cathepsin D is a protease involved in invasion of the cancer and metastasis formation. The purpose of the study was to evaluate a prognostic value of cathepsin D activity in blood serum and urine of patients with cancer of the stomach, pancreas and liver. The study was carried out on the samples of blood serum and urine obtained from patients with cancer of the stomach, pancreas and liver treated surgically at the First Department of General Surgery and Endocrinology of the Medical University of Białystok. The control group consisted of healthy individuals. Activity of cathepsin D was determined in serum and urine by the Folin-Ciocaltau method with the cupric modification and was expressed in nmol Tyr/ml/6h. Specific activity of cathepsin D was determined in the urine, and was expressed in nmol Tyr/mg of protein/6h. Protein concentration in serum was assessed with Lowry et al. method and results were expressed in mg/ml. A significant increase in activity of cathepsin D in serum (p = 0.0169) and urine (p = 0.0008) and an enhanced specific activity in the urine (p = 0.0085) was found in patients with cancer of the pancreas as compared with the controls. A significantly increased activity of cathepsin was revealed in serum (p = 0.0233) of patients with cancer of the stomach. No significant differences of cathepsin D activity were found in urine of the patients with cancer of the stomach when compared to the controls. Additionally, an upward tendency (almost two-fold increase) of cathepsin D activity was shown in blood serum and an increase in the activity and in specific activity was observed in urine of both patients with cancer of the liver in comparison with the healthy individuals. There were no significant differences in the activity of cathepsin D in serum of the patients with cancer of the pancreas and stomach (p = 0.4156). A statistically significantly higher activity (p = 0.0004) and specific (0.0048) cathepsin D activity was demonstrated in urine of the patients with cancer of the pancreas in comparison with the patients with cancer of the stomach. Determination of protein level in urine proved a downward tendency in the patients with cancer of the stomach (p = 0.11109), as compared to the controls, and a statistically significant increase found in the patients with cancer of the pancreas (p = 0.0238), in comparison with the patients with cancer of the stomach. In conclusion, investigation of cathepsin D activity in the blood serum of patients with cancer of the stomach and pancreas and in the urine of the patients with cancer of the pancreas may be usefull in clinical oncological diagnostics. However, further studies of the enzyme are necessary to establish the clinical value of cathepsin D measurement.