Congenital disorders of glycosylation-Ia are the most frequent type of congenital disorders of glycosylation. This condition affects the nervous system as well as other organs. The estimated incidence of congenital disorders of glycosylation-Ia is higher than the number of identified cases, therefore underdiagnosis of this heterogeneous disorder is probable. Neurologic and biologic signs are hallmarks for the identification of patients with congenital disorders of glycosylation-Ia. This report describes two children with congenital disorders of glycosylation-Ia syndrome confirmed by phosphomannomutase gene mutations with normal development and absence of biologic anomalies such as elevated transaminases and altered hemostasis. In conclusion, congenital disorders of glycosylation should be considered in cases of unexplained behavioral symptoms such as hyperactivity and concentration difficulties and mild neurologic signs. Intellectual retardation is often overestimated because of dysarthria and motor difficulties. Psychomotor reeducation might improve quality of life.