Lactose malabsorption was assessed by the hydrogen breath test in 40 Italian patients with irritable bowel syndrome and 42 controls without abdominal disturbances. Sixty-five percent of patients were "low milk consumers" (0-250 ml milk per day) compared with 38% of controls (P less than 0.02). Lactose loads of 25 and 50 g caused malabsorption in 82.5 and 87.5% patients and in 55 and 62% controls, respectively (patients vs controls P less than 0.02). Malabsorption was more frequent in the "low milk consumers" group (P less than 0.05). During a four-month lactose-free diet as the only treatment 7.5% of patients became symptom-free (and remained so for a further eight-month diet), 52.5% improved, and 40% showed no change.