Ciliary neurotrophic factor is the first neurotrophic factor to show survival-promoting effects in developing motor neurons in vitro, in ovo, and in vivo. In the present study we tested the effects of recombinant rat or human ciliary neurotrophic factor in the wobbler mouse model of motor neuron disease. Mice received 1 mg/kg of the factor or a vehicle solution subcutaneously three times a week for 4 weeks, after the disease was diagnosed between the ages of 3 and 4 weeks. Although treatment with rat ciliary neurotrophic factor (n = 6) resulted in delayed weight gain (p < 0.001), grip strength normalized to body weight in the factor-treated mice was significantly greater (p < 0.02) and declined at a slower rate (p < 0.05) compared to that in vehicle-treated animals. Human ciliary neurotrophic factor (n = 27) produced no change in body weight and reduced paw position and walking pattern abnormalities (p < 0.001 and p < 0.02, respectively). After 4 weeks of treatment, the mean grip strength of human ciliary neurotrophic factor-treated animals was twice as great (p < 0.001) and declined at a much slower rate (p < 0.005) than that of control mice. The time required to run 2.5 ft was less (p < 0.005) and muscle twitch tension was greater (p < 0.002) in ciliary neurotrophic factor-treated animals. Thus, ciliary neurotrophic factor retarded the disease progression and improved muscle strength in this motor neuron disease model.