Ciliary neurotrophic factor (CNTF) maintains survival of adult motor neurons. Mice lacking the CNTF gene develop mild, progressive motor neuron loss. In the normal human population, 1 to 2.3% are homozygous for a null allele, and reports suggest this mutant is associated with a younger onset of amyotrophic lateral sclerosis (ALS). We have tested this hypothesis in a study of 400 subjects with ALS and 236 controls. There was no difference in age of onset, clinical presentation, rate of progression, or disease duration for those with one or two copies of the null allele, excluding CNTF as a major disease modifier in ALS.