Carbohydrate-deficient glycoconjugate (CDG) syndrome type I due to phosphomannomutase deficiency (CDGIA) is the most common among a group of metabolic disorders characterized by a defective glycosylation of glycoconjugates. Clinically it is a multisystem disease with an important involvement of the central nervous system including pontocerebellar atrophy. Here the developmental patterns and results of neuropsychological assessment of four young adults with CDGIA syndrome are reported. The patients, aged 14-26 years, had classical clinical findings of CDGIA syndrome and olivopontocerebellar atrophy of severe degree. They had a marked delay in all areas of psychomotor development and gained to walk with aid, perform manipulative abilities and develop a communicative language after the 7th year. Later on, the acquired abilities remained stable, while self-help skills gradually improved, allowing the patients to join the family life. On neuropsychological assessment, there was mental retardation of variable degree with a special impairment of visuoperceptual skills, visuospatial organization, eye-hand coordination, verbal memory and language. Such findings, may be partially explained by the supratentorial atrophy in our patients and add more evidences to the role of the cerebellum and brainstem in the acquisition of non-motor cognitive functions. This study expands our understanding on the clinical spectrum of CDGIA syndrome and may be helpful for planning rehabilitation and education.