This report describes the use of metyrosine (Demser) in an adolescent male with psychosis associated with the 22q11.2 deletion syndrome (velocardiofacial syndrome; VCFS), diagnosed by fluorescence in situ hybridization (FISH). He presented with multiple features of 22q11.2 deletion syndrome, including ventricular septal defect, palatal abnormalities, speech and motor delays, attention deficits, mood lability, and psychosis. After a failed trial of an atypical antipsychotic to address the psychosis, metyrosine was initiated, with significant reduction of psychotic symptoms and mood lability. Metyrosine treatment allowed this youth to live at home and to attend school, after months of recurrent psychiatric hospitalizations. The successful treatment of metyrosine for psychosis associated with VCFS represents a first in psychiatry, where a known biochemical abnormality in a psychiatric disorder was corrected by a treatment that targets the biochemical pathway, leading to reduction of psychiatric symptoms and improvement of functioning.