Background: Vitamin D is important for the brain development, especially during the prenatal and early neonatal period. We aimed to study the vitamin D supplementation effect on behavior and language development, as an adjuvant therapy to the usual protocol of treatment, in attention deficit hyperactivity disorder (ADHD) children.Patients and Methods: This case- control prospective interventional study was carried out on 30 children aged from 3 to 7 years old, both sexes diagnosed with and delayed language development. Patients were divided into two equal groups: Group 1: Case group, and group 2: Control group. All patients were undergone general examination, vocal tract examination, history taking, ear and nose examination, auditory perceptual assessment of voice and speech, the conners parent rating scale, the modified preschool language scale -4 (PLS4) and the Egyptian Arabic Pragmatic Language Test (EAPLT).Results: There was significant improvement in cases before compared to cases after, and control before and control after regarding inattention, hyperactivity, and psychosomatic problems. At the study end, a significant decrease was found in cases after contrasted to control after regarding inattention, and psychosomatic problems. Concerning the scores of the language assessment, there was significant increase in cases after compared to cases before, and control after and control before regarding all scores of the PLS4, and EAPLT (P < 0.05).Conclusion: Vitamin D supplementation could be combined with behavioural modification therapy and language therapy for much more alleviation of ADHD symptoms and enhancing language development in ADHD children with language delay.