Fifty-seven patients with amyotrophic lateral sclerosis (ALS) were randomly assigned to receive single subcutaneous doses of 0.5, 1, 3, 7, 10, or 30 μg/kg recombinant human ciliary neurotrophic factor (rHCNTF) or placebo. Peak plasma concentrations occurred 180 to 260 min after dosing. rHCNTF plasma concentrations then appeared to decay, with an apparent elimination half-life of 120–400 min. Eight of 12 patients who received the highest dose had measurable plasma rHCNTF levels 24 h after dosing. Peak plasma concentrations of rHCNTF in patients receiving 30 μg/kg rHCNTF were in the range that has been shown to support the survival of embryonic rat and human motor neurons in tissue culture. Systemic exposure to rHCNTF increased with increasing dose. The elimination of rHCNTF, over the dose range evaluated, appeared to follow first-order processes. Elevations in body temperature and evidence of activation of the acute phase response were observed.