Levels of neurofilament subunits, potential biomarkers of motor axon breakdown, are increased in amyotrophic lateral sclerosis (ALS) patient's CSF but data on blood are not available. We measured blood levels of the phosphorylated axonal form of neurofilament H (pNF-H) by ELISA in transgenic rodent models of superoxide dismutase 1 (SOD1) ALS, and in 20 ALS patients and 20 similar aged controls monthly for 4 months. All symptomatic rodent ALS models showed robust levels of blood pNF-H, while control rodents or mice transgenic for unmutated SOD1 showed no detectable blood pNF-H. Average pNF-H levels in the G93A SOD1 mouse progressively increased from day 74 through death (day approximately 130). Median blood pNF-H level in ALS patients was 2.8-fold higher than controls (p < 0.001). Median ALSFRS-R declined a median of 0.8 pt/month (p < 0.001); higher baseline pNF-H level appeared to be associated with faster ALSFRS-R decline over 4 months (p = 0.087). The median rate of decline in ALSFRS-R was 1.9 pt/month in patients with baseline pNF-H levels above the median pNF-H value of 0.53 ng/mL; ALSFRS-R declined at a median of 0.6 pt/month in patients below this level. The pNF-H levels were relatively stable month to month in individual patients, raising questions regarding the molecular pathogenesis of ALS. Baseline control human pNF-H levels were higher in men than women and increased minimally over time. These data suggest that blood pNF-H can be used to monitor axonal degeneration in ALS model rodents and support further study of this protein as a potential biomarker of disease prognosis in ALS patients.