Ciliary neurotrophic factor (CNTF) is a cytokine that has been reported to affect cellular differentiation. Mouse CNTF knockouts have progressive motor neuron atrophy, but this protein has uncertain physiological function in humans. A naturally occurring CNTF variant in man, observed in many populations, abolishes function of the protein product, providing the opportunity to study loss of CNTF function in humans. It has been reported previously that this variant does not predispose to several neurological and neuropsychiatric disorders, including Alzheimer's disease, but findings have been more ambiguous with respect to other conditions, such as schizophrenia. We report here allele frequencies for this null mutation in populations diagnosed with mood disorders (unipolar depression, single episode or recurrent; N = 59), schizophrenia (N = 66), or Alzheimer's disease (N = 93). We found no association of the CNTF null with any of these phenotypes. There is presently no known phenotype consistently associated with either heterozygosity or homozygosity for the CNTF null allele, suggesting either that this protein does not serve a necessary function in humans or is redundant with some other protein or that any human phenotype associated with absence of CNTF is considerably more subtle than that seen in mouse.