Nusinersen (previously ISIS-SMNRx) is an antisense oligonucleotide designed to bind to the SMN2 pre-mRNA and promote inclusion of exon 7. This first-in-human, open-label, single- ascending dose study was designed to examine safety, tolerability, pharmacokinetics, and preliminary clinical effects of intrathecal nusinersen in medically stable patients (aged 2-14 years) with type 2 and type 3 spinal muscular atrophy (SMA). Four ascending single-dose levels (1, 3, 6, and 9 mg) were examined in cohorts of 6-10 participants. Participants were monitored for safety and tolerability, and CSF and plasma pharmacokinetics were measured. Exploratory efficacy endpoints included the Hammersmith Functional Motor Scale Expanded (HFMSE) and Pediatric Quality of Life Inventory. A total of 28 participants enrolled in the study (n = 6 in first 3 dose cohorts; n = 10 in the 9-mg cohort). Intrathecal nusinersen was well-tolerated with no safety/tolerability concerns identified. Plasma and CSF drug levels were dose-dependent, consistent with preclinical data. Extended pharmacokinetics indicated a prolonged CSF drug half-life of 4-6 months after initial clearance. A significant increase in HFMSE scores was observed at the 9-mg dose at 3 months postdose (3.1 points; p = 0.016), which was further increased 9-14 months postdose (5.8 points; p = 0.008) during the extension study. Results from this study support continued development of nusinersen for treatment of SMA.