Phosphomannose isomerase (PMI) deficiency (CDG‐Ib) is a newly recognized disorder of mannose and glycoprotein metabolism. PMI deficiency manifests itself mainly as a gastrointestinal disorder with protein‐losing enteropathy and life‐threatening intestinal bleeding. Hypoglycaemia is an additional prominent symptom. In contrast to phosphomannomutase deficiency (CDG‐Ia), there are no neurological symptoms. PMI deficiency blocks the endogenous mannose formation from glucose. Exogenous oral mannose supply bypasses the enzymatic block and leads to the disappearance of all symptoms in the patient. The striking ultrastructural abnormalities of the rough endoplasmatic reticulum of the duodenal epithelial cells completely normalize and the hypoglycosylation disappears, as evidenced by the normal isoelectric focusing pattern of serum transferrin, the standard diagnostic procedure for recognition of CDG. This paper includes a detailed description of the clinical symptomatology of the first‐ever diagnosed and treated patient with PMI deficiency and a 5–y follow‐up study of mannose therapy.