Lactose malabsorption, by the breath hydrogen test, and lactose intolerance (presence of symptoms) were studied in twenty healthy Italian subjects after intake of 12.5, 25 and 50 g lactose, whole milk and low-lactose milk. A rise in respiratory concentration of hydrogen (greater than 20 ppm) (malabsorption) was found in fifteen subjects after 50 g lactose, in thirteen after 25 g and in seven after 12.5 g. Symptoms generally occurred in subjects presenting a rise in respiratory hydrogen excretion, but such a rise was often observed without symptoms. Thirteen subjects presented symptoms after 50 g lactose, but only three after 25 g and one after 12.5 g. Whole milk (500 ml) gave a lower incidence of lactose malabsorption than 25 g lactose (7/20 versus 13/20, P less than 0.05) and more subjects developed symptoms (7/20 versus 3/20, NS). Low-lactose milk produced no malabsorbers and one intolerant. Breath methane was detected constantly in seven subjects and in three on some of the days of observation. Respiratory methane excretion generally appeared to be unrelated to lactose ingestion.