The case of a mentally retarded girl with a number of dysmorphic features, Raynaud's phenomenon, hypotonia and petit mal seizures is presented. Laboratory investigations showed alpha-aminoadipic aciduria and a high level of fetal haemoglobin. Oral L-lysine loading resulted in a marked increase of alpha-aminoadipic acid in blood and urine. After 3 months of pyridoxine medication the increase of alpha-aminoadipic acid in blood and urine during the oral L-lysine loading test was less than in the test before treatment. A normal degradation rate of DL-alpha-amino [1-14C] adipic acid in fibroblasts of the patient, as measured by 14CO2 production, did not indicate a primary enzyme defect in the alpha-aminoadipic acid transamination or decarboxylation steps. The persistent HbF could be the result of stress on the erythropoiesis by a secondary induced defect in an early stage of haemoglobin synthesis in which alpha-amino-beta-ketoadipic acid, a structural analogue of alpha-amino-adipic acid, is an intermediate.