The urea breath test was developed for the diagnosis of Helicobacter pylori infection. In this test, we measure exhaled ¹³CO₂ using POCone developed in Japan, which is an infrared spectrometry photometer. Based on the urea breath test, we subsequently developed the in vitro ¹³C-glucose exhaling test. In this test, ¹³C-glucose is added in the circulation culture medium, and ¹³CO₂ generated with a bioartificial liver is measured by the POCone. The quantity of discharged ¹³CO₂ reflects the energy-producing ability of the bioartifi- cial liver. Therefore, we can simply and easily observe the viability of a bioartificial liver and the effect of drugs on carbohydrate metabolism in the bioartificial liver in real time. According to the results of a bioartificial liver study, we developed the fasting ¹³C-glucose breath test, which can quickly determine hepatic insu- lin resistance in humans with marked sensitivity. The bioartificial liver study unexpectedly led to the development of a new stable isotope breath test. [Review].