The main aim of this study was to create a semiempirical model, helpful in estimating severity of the Helicobacter pylori (H. pylori) infection by using the urea breath test (UBT), when urea labelled ¹⁴C has been used for diagnostics. The model consists of four compartments representing stomach (1), blood vascular system (2), lungs (3) and urinary system (4). Mathematical model is based on the balance of radioactive ¹⁴C in compartments from 1 to 4. The histological investigations were used as reference methods. Comparison of the results obtained from simulation, which yields dependence of ¹⁴C activity on time, to experimental results of UBT, made it possible to determine the ranges of coefficient h_B value, which characterized each degrees of severity of H. pylori infection: degree 0 (lack of infection) – h_B below 0.025; degree 1 (not large) – h_B in range 0.025-0.115; degree 2 (moderate) – h_B in range 0.115-0.300; degree 3 (significant) – h_B above 0.300. It was possible to estimate severity of H. pylori infection in clinical practice on the basis of comparing the 14C activity value of experimental points as obtained from the breath test, to the results of simulation with suitable value of the fitted parameter h_B indicating degree of severity of infection.