Adenylosuccinate lyase (ADSL) deficiency is a rare disease of de novo purine synthesis. The main symptoms are psychomotor retardation, epilepsy, autistic features, occasionally associated with muscular hypotonia. Diagnosis is made by detection of abnormal purine metabolites (succinyladenosine - S-Ado and succinylaminoimidazole carboxamide riboside - SAICAr) in body fluids. The severity of the clinical features correlates with low S-Ado/SAICAr ratio. We report clinical, biochemical and brain MRI findings of a female infant with severe early epilepsy and hypotonia, who died at the age of 10 weeks.