text: This study examined the clinical history of the CSF and urine of two Greek siblings who were both diagnosed with SR deficiency. Both siblings displayed delayed psychomotor development and a movement disorder. The diagnosis was confirmed by measuring the SR enzyme activity and mutation analysis. The mutation analysis of the gene was performed using genomic DNA isolated from blood samples. The results concluded that both patients have low concentrations of HVA and HIAA and high concentrations of sepiapterin in the CSF, but neopterin and biopterin were abnormal in only one sibling. The results of this research indicates that when diagnosing the SR deficiency, the quantification of sepiapterin in the CSF is more important and indicative of SR deficiency than using neopterin and biopterin alone. The results also show that the urine concentrations of neurotransmitter metabolites are abnormal in patients with this disorder. This finding may provide an initial and easier indication of the deficiency before CSF analysis is performed.