Elevated activities of cysteine proteinases in cancers are attributed to impaired regulation by the endogenous cysteine proteinase inhibitors (cystatins). Cystatin C is suggested to be a reliable marker of glomerular filtration rate (GFR). The purpose of this study was to evaluate whether cystatin C concentration was influenced by gastric malignancy, which is a common cancer type and the availability of this parameter safely for screening renal dysfunction in these patients. Since there is inadequate information on the clinical significance of cystatin C expression in human gastric cancers, we studied the differences in levels of serum cystatin C in this type of cancer and also evaluated them with respect to cancer stages. The levels of cystatin C in patients with gastric adenocancer were similar to those in the control group. Similarly, the values of serum creatinine and GFR in the patients were comparable to those of the controls. Serum creatinine values in controls and in patients did not show a statistically significant difference 0.8 +/- 0.16 and 0.9 +/- 0.35 respectively (p> 0.05). Serum cystatin C values in controls and patients were 0.74 +/- 0.32 and 0.72 +/- 0.6 respectively, which was also not different statistically (p> 0.05). When the patients were evaluated with respect to the stages of the cancer, the cystatin C values did not differ significantly. The mean +/- SD levels of the patients in low and advanced stages were 0.79 +/- 0.51 and 0.69 +/- 0.64 respectively (p> 0.05). Our study clearly demonstrates that serum cystatin C has valuable potential for the detection and monitoring of GFR and may be safely used in gastric cancer patients.