Objective: Urotensin-II (U-II) is one of the most vasoconstrictive substrates for the mammals. Lately, this substrate is thought to be responsible for developing of the neuropsychiatric disorders, by causing an abnormal brain bloodstream situation. Autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are frequently seen disorders in childhood and their etiologies are remain unclear. This study evaluated the serum urotensin-II levels of children with ASD and ADHD and compared with healthy subjects' urotensin-II levels. Methods: Total of 179 children between age of 4-12, 60 of them diagnosed with ADHD and 60 of children with ASD, according to the DSM-5 criteria and both had no treatment for at least a month and 59 of healthy subjects whom they all admitted to the Ankara Pediatric Hematology-Oncology Training and Research Hospital were included. Schedule for Affective Disorders and Schizophrenia for School-Age Children, a semi-structured interview, was applied to all subjects. Venous samples of the participants were given after a-12 hours starvation. Serum U-II levels were analyzed by the use of ELISA kits. SPSS 16.0 was used for analysis and p [Anadolu Psikiyatri Derg 2018; 19(1.000): 80-86]