To identify antecedent events contributing to the development of amyotrophic lateral sclerosis, we studied 25 amyotrophic lateral sclerosis patients in whom we tabulated the incidence of factors previously associated with motor neuron disease and compared the incidences with those found in 25 hospitalized patients and 25 normal people. More amyotrophic lateral sclerosis patients reported exposure to lead and mercury, participation in athletics, and consumption of large quantities of milk. Exposure to lead and mercury, athletic participation, and milk ingestion are possible risk factors that may predispose to the development of amyotrophic lateral sclerosis.