The disease course and therapy of a nine-and-a-half-year-old boy with hyperphenylalaninemia due to a dihydropteridine reductase deficiency are reported. Clinically, there is a marked mental retardation and complex basal ganglion symptoms. The cranial computed tomographic investigation shows bilateral, symmetrical, comma-shaped calcifications in the globus pallidus and the putamen of the lentiform nucleus. The cause of these basal ganglion calcifications remains unclear. Lowering of serum and CSF folic acid levels could not be detected, in contrast to cases with the same enzyme defect described previously.