Congenital glycosylation defects are autosomal recessive disorders clinically characterized with growth retardation, hypotonia and multisystemic involvement. Congenital glycosylation defect type Ib is due to deficiency in phosphomannose isomerase which converts fructose-6-phosphate into mannose-6-phosphate. Patients usually present with hepatic or gastrointestinal symptoms lacking cranial involvement, making their IQ completely normal. We report a 10-month-old female patient referred to our clinic with persistent hypoglycemia, failure to thrive and hepatosplenomegaly who was diagnosed with congenital glycosylation defect type Ib. Oral D-mannose therapy was initiated shortly after diagnosis and her symptoms resolved in two weeks. Congenital glycosylation defect type Ib is an easily treatable disease and should be kept in mind in differential diagnosis in children and adults who show gastrointestinal symptoms, hyperinsulinemic hypoglycemia, palpable liver and spleen, growth retardation and elevated liver function tests.