目的对一例经典型苯丙酮尿症合并21-三体综合征患儿的临床特征进行研究。方法采用BH4负荷试验结合尿喋呤谱和红细胞二氢喋啶还原酶(DHPR)活性测定的方法对高苯丙氨酸血症进行诊断分型;G显带技术分析外周血染色体核型。结果①患儿诊断为经典型苯丙酮尿症。②患儿外周血染色体核型为47,XX,+21,诊断为21-三体综合征。结论苯丙酮尿症可能合并有其他遗传代谢性疾病,临床诊断须全面。 Objective To study the clinical features of a classic case of phenylketonuria in children with trisomy 21 syndrome. Methods The phenotype of hyperphenylalaninemia was detected by BH4 load test combined with the analysis of the urinary excretion profile and DHPR activity. The G banding technique was used to analyze the karyotypes of peripheral blood. Results ① Children diagnosed as classic phenylketonuria. ② children with peripheral blood karyotype 47, XX, +21, diagnosed as 21-trisomy syndrome. Conclusion Phenylketonuria may be associated with other genetic metabolic diseases, clinical diagnosis should be comprehensive.