Abstract: In the central nervous system (CNS), neuregulin-1 (NRG-1) proteins function in neuronal migration, differentiation, and survival of oligodendrocytes. The NRG-1 gene codes for at least 15 different isoforms, which may be classified on the basis of their molecular structure. At least two different haplotypes of the NRG-1 gene may be associated with schizophrenia. An abnormal expression pattern of NRG-1 mRNA was found in the prefrontal cortex of schizophrenic patients in comparison to controls. We here show that the NRG-1α isoform is significantly reduced in white matter of the prefrontal cortex in schizophrenia but not in affective disorder. In the prefrontal gray matter, the density of NRG-1α expressing neurons was reduced in individuals with schizophrenia and in unipolar patients. We studied brains of 22 schizophrenics, 12 patients with affective disorders (7 unipolar and 5 bipolar), and 22 matched controls. NRG-1α immunoreactive material was detected with a polyclonal antiserum against the synthetic peptide from α-type EGF-like domain of human NRG. The demonstrated decreased number of NRG-1 immunoreactive neurons in the brains of schizophrenics and patients with unipolar depression points to an important role of this NRG-1α splice variant in neuropsychiatric disorders. Reduced NRG-1α protein concentrations were found in brains of schizophrenics after Western blot analysis. The diminished expression of NRG-1α strongly supports an early neurodevelopmental component to schizophrenia.