To investigate the prevalence of lactose malabsorption among patients with functional gastrointestinal disturbances we prospectively evaluated all patients referred to a gastrointestinal outpatient clinic over a period of 18 months. All patients had a breath hydrogen test following oral lactose in addition to the standard diagnostic procedures. In 37 of the total of 64 patients no organic cause of the gastrointestinal complaints was found. In 9 of these 37 patients (24%) the breath hydrogen test indicated lactose malabsorption. Three to 6 month later most of the patients with lactose malabsorption showed a significant reduction of gastrointestinal complaints after they had maintained a lactose-poor diet. In comparison, patients with functional disturbances but without lactose malabsorption reported nor or only minor improvement of symptoms; most of these patients had consulted another physician since the last visit in the clinic.