Congenital disorders of glycosylation (CDG) are recessively inherited multisystemic disorders resulting from several genetic defects affecting the assembly, transfer or processing of oligosaccharides onto proteins and other glycoconjugates. CDG type Ib is due to a deficiency of phosphomannose isomerase (PMI) encoded by the MPI gene. PMI catalyzes the interconversion of fructose-6-P and mannose-6-P. The clinical phenotype is characterized by gastro-intestinal and hepatic symptoms. In contrast to most CDG patients, there is no neurological affectation. It's a mannose treatable disorder. We report the first recognised case of CDG Ib in Spain. He presented at 6 months with hypoglycaemia, failure to thrive and hypertransaminasaemia. He subsequently developed an enteropathy with subtotal villous atrophy on biopsy. The %CDT was very high and he presented with a type 1 pattern in transferrin isoelectric focusing. PMI activity in fibroblasts was very deficient. Mutations in MPI gene at R219Q and R56fs were found. Clinical and biochemical parameters normalised after treatment with mannose 1 g/kg/day in 5 doses. CDG Ib should be considered in patients with hypoglycaemia, liver disease, enteropathy and hypercoagulability, in the absence of other common causes, and particularly if some of them are combined.