The congenital disorders of glycosylation (CDG) are autosomal recessive disorders of N-glycans processing. Several different subtypes have been identified in recent years. Cerebellar atrophy is a characteristic finding in subtype Ia. We report clinical, imaging and genetic findings in a patient with a particularly benign clinical course, who had a normal CT at the age of 9 months and a new, previously undescribed, combination of mutations of the PMM gene locus on chromosome 16p13 (647,691). The 691 mutation has been described only in severe cases so far. This could indicate that genotype-phenotype correlation is lower than expected.