A 10-month old girl is described with a serum transferrin isoform abnormality of the same kind as in two previously reported girls with carbohydrate-deficient glycoprotein syndrome type III. This patient presented with joint abnormalities and rapidly developing hypsarrhythmia, hypotonia, psychomotor delay and growth retardation. Fingers, toes, nails and local skin were dysmorphic. She had pale optic discs, thoracic syringomyelia and frontal lobe atrophy at three months. The CDT value in serum was greatly elevated. Several carbohydrate-deficient isoforms were found in transferrin (four), alpha1-antitrypsin (three), antithrombin (two) and thyroxine-binding globulin (four). Mutations in the CDGS 1-gene were excluded. The CDGS III glycoprotein abnormality most probably represents a distinct disorder of glycoprotein metabolism, and needs to be considered in unclear hypsarrhythmia with developmental delay. Dysmorphic features may be added to this syndrome.