Abstract: Objective：Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine (Phe) metabolism resulting from deficiency of phenylalanine hydroxylase (PAH).Fewer phenylketonuria results from deficiency of tetrahydrobiopterin (BH4)，called malignant phenylketonuria，which was often misdiagnosed for its variant phenotype.Method：A malignant phenylketonuria patient presented with paroxysmal movement disorders was studied via blood phenylalanine measurement，urine metabolism screening，phenylalanine and tetrahydrobiopterin stress test，and urine petrin analysis.A comprehensive analysis was made based on literature.Result：A little one-year-and-six-month old girl presented with paroxysmal movement disorders;her blood phenylalanine concentration was normal;urine metabolism screening suggested phenylketonuria.She was diagnosed with malignant phenylketonuria according to phenylalanine and tetrahydrobiopterin stress test and urine petrin analysis.Conclusion：There is a high incidence and misdiagnosis rate for malignant phenylketonuria in China，so more attention should be paid to it in clinic.

Key words: Phenylketonuria/Diagnosis；Tetrahydrobiopterin/Lack；Paroxysmal movement disorder, Epilepsy, Gene mutation