AbstractGlycogen storage disease type I (GSD I) (McKusick 232200) is caused by inherited defects of the glucose‐6‐phosphatase complex. Patients with GSD Ia as well as patients with GSD Ib may suffer from intermittent diarrhoea, which seems to worsen with age. The cause of this diarrhoea is unknown. This study describes the results of investigations of intestinal functions and morphology in patients with GSD Ia and GSD Ib, which were performed to detect a common cause for chronic diarrhoea in GSD I. The following were investigated: faecal fat excretion, faecal α1‐antitrypsin and faecal chymotrypsin, expiratory H2 concentrations, persorption of cornstarch in urine and colonic biopsies. With the investigations presented in this study, no common cause for diarrhoea in GSD I was found. In GSD Ib loss of mucosal barrier function due to inflammation, documented by increased faecal α1‐antitrypsin excretion (3.5–9.6 mg/g dry faeces) and inflammation in the colonic biopsies, seems to be the main cause. The inflammation is most likely related to disturbed neutrophil function, which is often found in GSD Ib. Whether another cause is involved in GSD Ia and in GSD Ib, related to the disturbed function of glucose‐6‐phosphatase in the enterocyte, remains to be investigated.