13C-urea breath tests were sampled in 51 subjects with an isotope-selective non-dispersive infrared spectrometer (NDIRS) and compared with the results of conventional isotope-ratio mass-spectrometry (IRMS). Delta-over-baseline values of both methods correlated well (r = 0.976 at 15 min and 0.985 at 30 min, both p < 0.0005). With a cut-off for delta-over-baseline values of 5/1000 (i. e., 5 per thousand), both methods correctly classified all 36 Helicobacter-pylori-negative and all 15 H-pylori-positive subjects. All infected subjects had values over 11/1000 in all samples with both methods. NDIRS is of equal value as IRMS for the diagnosis of H pylori infection. Because NDIRS gives on-line results and is easier and cheaper, we consider NDIRS useful for clinical practice.