A new device for estimation of hydrogen in expired air is described. The measuring principle consists of a semi-conductive detector with a high affinity for hydrogen. Experiments on reliability and reproducibility are satisfactory. A good correlation has been obtained with the results of gas chromatographic analysis of hydrogen in reference gases as well as in samples of expired air. Discrimination between lactose absorbers and lactose malabsorbers in 50 consecutive patients is the same when using both methods simultaneously. This so-called "Lactoscreen", supplied with separate collection systems for infants and for older children, provides a rapid estimation of breath hydrogen concentration. A built-in hydrogen generator produces hydrogen air mixtures of variable concentrations, thus obviating the need for external reference gases. The "Lactoscreen" appears to be reliable for routine screening for carbohydrate malabsorption in children and adults and is easy for medical assistants to handle.