A total of 1805 apparently healthy, adult and adolescent Germans (1572 males and 233 females with a mean age of 20.3 years) were examined for lactose absorption capacity employing a field version of the breath hydrogen (H2) test. The diagnostic parameter, maximal change of breath hydrogen concentration 120 or 150 min after a load of 50 g lactose, showed a bimodal distribution, separating lactose absorbers (n = 1537, 85.2%) and lactose malabsorbers (n = 268, 14.8%). The distribution of the adult lactase phenotypes was independent of age, sex, and educational status. The incidence of gastrointestinal symptoms after lactose administration demonstrated the incongruity of lactose malabsorption and lactose intolerance. In addition to grouping by residence, the probands were classified according to the birthplaces of their grandparents in order to reconstruct the distribution pattern of the lactase phenotypes prior to World War I, a period of relative population stability. Considerable differences in the frequency of lactose malabsorption were found in regions corresponding to traditional ethnic groups within the German population: northwest Germany 6-9%, west and south 13-14%, southwest 23%, east (including formerly German territories east of rivers Oder and Neisse) 22%. These differences are discussed with reference to population history. The present fairly even distribution of the lactase phenotypes in West Germany is the result of internal migrations at the end of World War II.