Tetrahydrobiopterin ($BH_4$) deficiency is caused by mutations in genes encoding enzymes involved in the synthesis and regeneration of $BH_4$. The condition is usually accompanied by hyperphenylalaninemia (HPA) and deficiency of neurotransmitter precursors L-dopa and 5-hydroxytryptophan. $BH_4$ deficiency is much rarer than classical phenylketonuria. Dihydropteridine reductase (DHPR) deficiency, an autosomal recessive genetic disorder, is a cause of malignant hyperphenylalaninemia due to $BH_4$ deficiency. When left untreated, DHPR deficiency leads to neurologic deterioration at the age of 4 or 5 months, including psychomotor retardation, tonicity disorders, drowsiness, irritability, abnormal movements, hyperthermia, hypersalivation, and difficulty swallowing. Treatment of DHPR deficiency should be initiated as early as possible with $BH_4$ supplementation and replacement of the neurotransmitter precursors L-dopa and 5-hydroxytryptophan. We report the first case of DHPR deficiency in Korea, a child diagnosed at 9 years of age by genetic testing.