The timing and yield of metabolic studies for patients with neurodevelopmental disorders is a matter of continuing debate. We determined the yield of additional or repeated metabolic studies in patients with neurodevelopmental disorders. Patients referred to a tertiary diagnostic center for patients with unexplained neurodevelopmental disorders were included. Initial metabolic studies had been performed in most patients (87%) before referral. Additional/repeated metabolic studies were individually tailored. Twelve metabolic diseases of 433 patients studied (2.8%) were diagnosed, despite normal initial metabolic studies before referral. Specific metabolic investigations lead to a greater diagnostic yield in patients with neurodevelopmental disorders. Ann Neurol 2008