Noonan syndrome is characterized by short stature, typical facial dysmorphology, and congenital heart defects. The main facial features of Noonan syndrome are hypertelorism with down-slanting palpebral fissures, ptosis, and low-set posteriorly-rotated ears with a thickened helix. The cardiovascular defects most commonly associated with this condition are pulmonary stenosis and hypertrophic cardiomyopathy. Other associated features are webbed neck, chest deformity, mild intellectual deficit, cryptorchidism, poor feeding in infancy, bleeding tendency, and lymphatic dysplasias. The patient is a 10-year-old boy. He had experienced repeated febrile convulsions. He had typical facial features, a short stature, chest deformity, cryptorchidism, vesicoureteral reflux, and mental retardation. His language and motor development were delayed. When he went to school, it was difficult for him to pay attention, follow directions, and organize tasks. He also displayed behavior such as squirming, leaving his seat in class, and running around inappropriately. Clinical observation is important for the diagnosis, so we report a patient who was diagnosed with Noonan syndrome, mental retardation, and attention-deficit hyperactivity disorder.