3-phosphoglycerate dehydrogenase deficiency is a severe but treatable disorder of serine synthesis, first described in 1996 (Jaeken et al. 1996a). The patient presented with West syndrome, severe psychomotor delay, failure to thrive, microcephaly, atypical ocular movements, and pyramidal signs. Treatment with oral L-serine abolished seizures and improved psychomotor development, hyperexcitability, head growth, cortical and subcortical hypotrophy, and hypomyelination of the brain on MRI scans. 3-phosphoglyce-rate dehydrogenase deficiency is a treatable congential error that probably leads to West syndrome.