We analyzed data from the 245-patient placebo group of the ALS CNTF Treatment Study Group study, a large, prospective, multicenter study of recombinant human ciliary neurotrophic factor to determine prognostic factors for length of survival in ALS. Variables examined included baseline demographic characteristics, indices of disease severity, pulmonary function, and clinical laboratory tests. Shorter survival was associated with greater age, lower percent-predicted forced vital capacity (FVC%), and lower serum chloride at study entry. A shorter interval from symptom onset to diagnosis of ALS and greater weight loss in the 2 months before study entry also predicted shortened survival times. The rate of muscle strength loss before study entry did not predict risk of mortality. Serum chloride, reflecting the degree of respiratory acidosis, was identified for the first time as being correlated with prognosis in ALS. The relationship between a patient's FVC% and the probability of survival is described.