Neurotrophin-3 (NT-3) may have a potential role in the pathogenesis of schizophrenia, given evidence of abnormal neurodevelopment in schizophrenia, as well as a potential association of an NT-3 gene polymorphism with schizophrenia. Cerebrospinal fluid NT-3 protein was assayed using an enzyme- linked immunosorbent assay in five patients with schizophrenia and 49 patients with medical illness. None of the patients with schizophrenia had detectable levels of NT-3 (above 4.7 pg/ml). Eleven samples from 10 different patients with medical or neurological illness had detectable levels of NT-3, associated with surgery for hydrocephalus or central nervous system infection.