A new device Tunable Diode Laser Spectroscopy (LS) was developed for the analysis of isotope ratios of 13CO2/12CO2. Its applicability for breath tests was validated. The exhaled breath is collected in a 2 l aluminum bag, of which CO2 is separated by cryogenic system and introduced into LS. Repeat measurements (8 times) of two kinds of gases (delta 13C:-2.62 and - 1.14%) revealed 0.045% and 0.065% variation (IS.D.), from which precision of measurement was estimated as 0.2% (3S.D.). Seven healthy volunteers were given orally different dose of 13C-glycine (50 mg-200 mg in 5 subjects) and 13C-methacetin (30 mg and 150 mg in 2 subjects). One patient with gastric ulcer with helicobacter pylori (HP) infection was given 100 mg of 13C-urea. Serial breath samples (9-15) were taken and measured both by LS and mass spectrometer (MS) thereafter. Appreciable peaks were obtained at 30-50 min. after glycine and methacetin ingestions. The height of the peaks were dose dependent. Increased excretion of 13CO2 characteristics to HP infection was obtained in 13C-urea breath test. Measured values with LS were identical with those measured with MS (60 samples) with the range of difference within 0.2% (3S.D.). [Y (LS) = 1.02X (MS) + 0.55, r = 0.996] We conclude the LS is suitable for various 13C-breath tests.