Genetic susceptibility plays an important role in the development of schizophrenic psychoses, and the neural maldevelopment hypothesis is suggested by neuropathological and neuroimaging findings. We investigated the association between a null mutation in the ciliary neurotrophic factor (CNTF) gene and functional psychoses including schizophrenia and schizoaffective disorder. The frequency of mutant allele was significantly increased in patients with schizoaffective disorder, but not in those with schizophrenia in comparison with controls. TheCNTFnull mutation resulting in CNTF deficiency may confer potential susceptibility to schizoaffective disorder.