Spinal muscular atrophy (SMA) is a rare neuromuscular disease caused by a homozygous deletion of the SMN1 gene. The resulting lack of survival motor neuron (SMN) protein leads to progressive muscle weakness and often respiratory insufficiency. There is unmet need for reliable biomarkers to predict disease severity, prognosis, and treatment effect in patients with SMA. Nusinersen is an antisense oligonucleotide that leads to an increased SMN protein production by the SMN2 genes and has been shown to be effective in children and adults with SMA. Previously, we have shown that neurofilament (NF) may serve as a distal pharmacodynamic and/or disease progression marker for SMA infants undergoing nusinersen treatment. However, changes in NF levels in SMA adults with nusinersen treatment have not yet been demonstrated. Currently there are no other established fluid biomarkers to evaluate disease progression or treatment response in SMA adults. In this study, we evaluated exploratory blood biomarkers in SMA adults at baseline and longitudinally following nusinersen treatment (up to 2.8 years): NF, glial fibrillary acidic protein (GFAP), and creatine kinase (CK). Plasma NF levels were similar across baseline SMA adults and age-matched controls, but NF levels were slightly decreased in response to a long-term nusinersen treatment. Serum GFAP and CK levels were slightly elevated in SMA adults relative to the age-matched controls. Upon nusinersen treatment, GFAP levels did not change, but CK levels decreased. Further research is needed to correlate NF and/or CK levels to clinical outcomes in SMA adults and validate their potential for clinical application.