Homocystinuria due to cystathionine β-synthase deficiency is an inborn error of metabolism first described almost 50 years ago, which involves the accumulation of plasma homocysteine and other metabolites. Without early detection and appropriate treatment, common and sometimes lethal consequences include ocular abnormalities, osteoporosis, developmental delays, marfanoid phenotype, vascular disease, and mental retardation. Almost 50 % of subjects develop a psychiatric disorder during their life, but only 2.8 % present a psychiatric symptom as the initial manifestation. Among this group, psychotic disorders are infrequent. We describe the case of a 17-year-old boy presenting with a first episode psychosis and an unknown homocystinuria due to cystathionine β-synthase deficiency, which led to a lethal outcome.