Patients who met International Congress of Gastroenterology criteria for irritable bowel syndrome (IBS) and had breath hydrogen lactose testing were interviewed to determine whether detection of lactose maldigestion (LM) had an impact on their symptoms. Of 199 patients initially evaluated, 161 (81%) were contacted and asked to rate their symptoms. At baseline, 47 (29%) of the IBS group had LM. Before testing, 23 (49%) were aware that ingestion of lactose-containing food was associated with their gastrointestinal symptoms. Lactose-maldigesting IBS subjects (IBSLM, n = 47) and those who had IBS and no LM (n = 114) were similar in terms of age, sex, and ethnic background. Interviews performed 41 +/- 1.1 (SEM) months after baseline evaluation revealed no significant differences in abdominal pain, altered bowel habits, bloating/distension, mucus, and relief with defecation among those with IBS or LMIBS. Overall symptoms resolved, improved, did not change, or worsened in a manner not statistically different between IBS and IBSLM groups. IBSLM subjects (a) felt that identifying LM helped them gain awareness of food-symptom relationships (78.7%), (b) experienced some improvement in symptoms (83%), (c) were avoiding lactose foods (87.2%), or (d) used lactase enzyme supplements (38.3%). Identifying LM did not significantly affect rated variables.