Significant developmental delay was first noticed when both sisters were in their third year of life. However, no biochemical disorders were found through the routine biochemical tests, including liver and kidney function, lipoprotein, urine and blood cell count analysis. Progressively, both sisters exhibited odd behaviour, accompanied by personality changes and altered sleep rhythm and then were diagnosed as attention deficit hyperactivity disorder. In the eighth year, the younger sister began to take risperidone due to a presumed psychiatric disorder. Four months before attending our hospital, both sisters were diagnosed by MRI as having hereditary leukodystrophy. Nerve-nurturing treatment was tried, but without good outcome. They were then referred to our hospital for further consultation. After systematic examinations, it was confirmed that both the sisters were suffering from phenylketonuria. The symptoms were alleviated after dietary restriction of phenylalanine and symptomatic treatment.