The measurement of hydrogen in exhaled air and changes in the concentration of blood glucose and urine galactose excretion are indirect methods of diagnosing hypolactasia. The aim of this study was to compare a portable breath hydrogen analyser (Micro H2) with a widely used model (Quintron MicroLyzer) and to compare them with the blood glucose, urine galactose, and gastrointestinal symptoms in the lactose tolerance test. After an overnight fast, 44 volunteers (18-66 y) ingested 50 g lactose in a single oral dose. Changes in exhaled breath hydrogen concentrations were measured with the two analysers, and changes in blood glucose and urinary galactose were assayed for 4 h and used as a reference. Eighteen subjects were diagnosed as maldigesters according to our gold standard of at least two positive tests out of the three: breath hydrogen by Quintron, blood glucose concentration, and urine galactose excretion. The highest increase in the breath hydrogen concentration over the baseline was highly variable: 44-366 ppm (Micro H2) or 27-187 ppm (Quintron MicroLyzer). The sensitivity, specificity, and positive and negative predictive values of the Micro H2 compared to the gold standard were 83%, 96%, 94% and 89%, respectively. Overall agreement was 91% (95% CI 78-97%). Compared to the Quintron, the diagnoses were identical in 100% of the cases (92-100%). Thus, for diagnosing hypolactasia, the Micro H2 appeared as reliable for measuring breath hydrogen concentrations as Quintron MicroLyzer commonly used in oral lactose tolerance tests.