AIM OF THE STUDY To investigate the effect of generation of tumor necrosis factor-alpha (TNFα) and the importance of TNFα(rs1 800629) gene polymorphism in the progression of chronic hepatitis C (CHC) and ulcerative colitis (UC). MATERIAL AND METHODS The study involved 90 patients with chronic hepatitis C, 50 patients with UC and 50 healthy donors. The blood concentrations TNFα and TNFα gene polymorphism (rs1800629) were evaluated. RESULTS TNFα levels in the blood in patients with chronic hepatitis C were increased compared with the control group and correlated with the severity of Cytolysis and fibrosis (r = 0.34, p = 0.02). At slow speed the formation of liver fibrosis TNFα amounted to 1.5 (0.9-2.8) pg/mI, with a fast speed--2.3(1.4-8.2) pg/mI (p = 0.006). Patients with UC at 3-4 degrees endo- scopic activity production of TNFα reached 6.5 (7-9) pg/mI, which was significantly higher than the value obtained at 1-2 degrees endoscopic activity--0.25(0-0.8) pg/ml (p = 0.001). The allelic variations of TNFα in groups of patients with CHC at different rates forming LF statistically differences were not found. The allele, associated with severe progressive course of UC and increased production of TNFα--A risk allele and genotype GA TNFα, associated with a slow progression of UC- "protective" G allele and genotype GG TNFα gene were determined. CONCLUSION Determining the level of TNFα allows to evaluate the severity of liver disease, heaviness and progression of liver fibrosis speed in CHC, and the severity of inflammation in the intestinal mucosa in UC. The presence of the allele A of TNFo(rs1800629) is a predictor of severe and progression of UC. Determining genetic polymorphism TNFα in patients with UC may be an additional factor to assess the prognosis of the disease.