The paper reports on experimental studies of the absorption of 13C16O2 in a mixture with 12C16O2 in exhaled air by laser absorption spectroscopy using a diode laser tunable in the frequency range 4860-4880 cm-1. The results of the studies showed that the minimum detectable concentration of 13C16O2 is at a level of less than 20 ppm, and this approach is promising for measuring the content of 13C16O2 in exhaled air for non-invasive online diagnosis of oncological diseases of the gastrointestinal tract at early stages