UNLABELLED: Recent evidence suggests that decline of regulatory T cells (Tregs) play a critical role in the prevalence of autoimmune diseases inhibiting the maintenance of peripheral self tolerance, while its augmentation leads to insufficient antitumor response, accompanied with poor prognosis in various malignancies. Increased number of Tregs (CD4+CD25+FoxP3+) were noticed in peripheral blood mononuclear cells (PBMCs), tumor-infiltrating lymphocytes (TILs) and/or regional lymph nodes lymphocytes (LNLs) of patients with gastrointestinal tumors. The aim of our study was to investigate the correlation between the percentage of Tregs in peripheral blood of patients with colorectal carcinoma, using flow cytometric technique and tumor stages, classified as Dukes' A, B, C or D and by stage of differentiation. Peripheral blood venous samples were obtained from 92 patients with colorectal cancer and from 30 healthy adult volunteers. STATISTICAL ANALYSIS: Linear regression equations were generated using a least-squares method and analyzed for differences of covariance. Statistical significance was calculated by Mann Whitney U-test. Our data has shown that 15% patients with colorectal cancer were classified as Dukes' A, 41% were Dukes' B, 35% were Dukes' C and 9% were Dukes' D. 54% patients with CRC were well differentiated, 11% were poorly differentiated, 20 were moderately differentiated, tage, 4% were mucinous carcinoma and rest of 11% were partly good differentiated with mucinous components. The increased percentage of Tregs in colorectal cancer patients correlates with tumor stage. These results indicate a possible involvement of regulatory T cells in disease progression. New strategies using inhibition or depletion of Tregs are necessary to elucidate the complexity of defective tumor immunity.