Background: Classical homocystinuria due to cystathionine b-synthase (CBS) deficiency (OMIM 236200) is a recessively inherited condition caused by mutations in the CBS gene. The founder mutation p.R336C accounts for almost all CBS deficiency in Qatar, affecting approximately 1 in 1,800 births, making it the most prevalent monogenic disease among the Qatari population. Untreated patients can have severe intellectual disability (ID), devastating multisystem complications and premature death. Current treatment is based on pharmacology therapy and life-long methionine-restricted diet, which is difficult to maintain particularly in late diagnosed individuals. Data on the neurodevelopmental and psychological impact of the disease on outcomes among Qatari patients are generally lacking and have not been studied.Objectives: To examine the cognitive, educational and psychological outcomes of classical homocystinuria on Qatari patients.Subjects and Methods: Thirty-two cases with classical homocystinuria and 25 sibling controls were recruited to evaluate the neurodevelopmental and cognitive outcomes. We reviewed the subjects' medical record and collected pertinent clinical and educational data from parents. Stanford-Binet Intelligence Test (Arabic translation -4th ed.) was used for cognitive (IQ) testing.