We investigated the safety and efficacy of the granulocyte colony stimulating factor (G-CSF) in 13 patients with amyotrophic lateral sclerosis (ALS). Five-day administration of 2 microg/kg once a day was followed by a six-month observation period. The primary and secondary endpoints were the changes of ALS functional rating scale (ALSFRS) and the compound muscle action potential (CMAP) amplitude, respectively. We found that the declines of ALSFRS and CMAP amplitude after G-CSF administration were significantly less than those measured prior to the treatment. The results suggest G-CSF is safe in ALS patients, and may affect the rate of motor decline.