Received Feb 27; Accepted May During period A, prior to the onset of disease, environmental exposures might act, alone or in conjunction with other factors, either to cause disease or to modify the risk and timing of disease onset. Telephone counseling has been used to overcome the logistical challenge of counseling those residing great distances from the study center. Schematic illustration of the full temporal course of the disease process in amyotrophic lateral sclerosis, showing different time periods during which environmental factors might be relevant During period A, prior to the onset of disease, environmental exposures might act, alone or in conjunction with other factors, either to cause disease or to modify the risk and timing of disease onset.