A Turkish girl presented with a history of fever, diarrhoea, convulsions, recurrent infections and failure to thrive from the age of 5 months. Megaloblastic anaemia was present and profound folate deficiency was evidenced in plasma and in CSF. Treatment with oral folic acid cured the anaemia, diarrhoea and infections but failed to prevent convulsions and the appearance of mental retardation and cerebral calcifications. Loading tests with folic acid and its derivatives led to the conclusion that the folate deficiency was caused by a defect in folate transport both across the gut and the blood-brain barrier. Low plasma concentrations of methionine prompted a therapeutic trial with methionine associated with vitamin B12 and folic acid that spectacularly improved the convulsions.