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The prevalence of lactose malabsorption (LM) in the Caucasian population of northern Europe is estimated to be low. Irritable bowel syndrome (IBS) is a very common diagnosis, and its symptoms are nearly identical to those of LM. Therefore we investigated the prevalence of LM among IBS patients in comparison with healthy volunteers.A double-blind clinical trial compared with healthy controls.One out-patient gastroenterology clinic in the Netherlands.70 Caucasian IBS patients and 35 healthy volunteers (staff members).All 105 underwent hydrogen (H2) breath and blood glucose tests, after an oral intake of 50 grams of lactose. The IBS patients were treated with a lactose-restricted diet for 6 weeks. They completed a lactose intake score before, and a symptom score scored by six separate criteria, before, during and after treatment.In 17 out of 70 (24.3%) IBS. patients LM was detected, in comparison with 2 out of 35 (5.7%) controls (P<0.009). There was no difference in the pre-entry mean lactose intake and symptom score between the LM positive and negative IBS patients. The mean symptom score of the LM positive group showed a marked decrease after 6 weeks of dietary therapy (P<0.001).A substantial number of IBS patients showed a clinically unrecognized lactose malabsorption, which could not be discriminated by symptoms and dietary history, and which can be treated with a lactuse-restricted diet. Therefore LM has to be excluded before the diagnosis IBS is made.