The causes of amyotrophic lateral sclerosis, Parkinson disease, and Alzheimer disease are unknown. Furthermore, treatment for two of these conditions is almost totally lacking. The thesis is presented that each of these disorders is due to lack of a disorder-specific neurotrophic hormone. The hormone would be elaborated or stored in the target of the affected neurons. It would be released by the postsynaptic cell and then exert its effects in a retrograde fashion after being taken up by the presynaptic terminal. In the lower motor neuron syndromes of amyotrophic lateral sclerosis, failure of muscle cells to release the appropriate motor neurotrophic hormone would result in impaired function of anterior horn cells. In Parkinson disease, the neurotrophic failure would be characterized by inability of striatal cells to provide the required dopamine neurotrophic hormone with resulting impairment of substantia nigra cells. In Alzheimer disease, the abnormalities would lie in failure of the hippocampus and cortical cells to supply the relevant cholinergic neurotrophic hormone with resulting impairment of medial septal and nucleus basalis neurons. Central nervous system tissue culture provides a convenient system in which to assay these neurotrophic hormones and should permit a test of the hypothesis.