The clinical significance of lactose malabsorption and the individual sensitivity to lactose were investigated in 20 patients with verified lactose malabsorption. Thirteen patients were relieved of all symptoms while seven improved only on a lactose-free dth lactose-free milk but following provocation with increasing amounts of lactose, the tendency to diarrhea and abdominal discomfort increased considerably. Three patients experienced discomfort after provocation with only 5 gl lactose. On provocation with increasing amounts of lactose the seven patients who had not recovered also developed increased abdominal discomfort but none of them developed increased tendency to diarrhea. It is concluded that, in addition to lactose malabsorption, these patients must suffer from ittitable colon with tendency to constipation.