In summary, our study demonstrates that (1) AQP4-IgG is detectable in the CSF in most AQP4-IgG seropositive NMOSD patients but not in that of patients with MS or OND; (2) that the presence of CSF AQP4-IgG in patients with NMOSD is positively associated with acute disease relapse within 30 days prior of LP; AQP4-IgG serum titres >1:250; and with blood-CSF barrier disruption; but not with treatment status or the type of acute clinical disease (myelitis or ON) at time of LP; (3) a lack of quantitative evidence for intrathecal synthesis of AQP4-IgG in most NMOSD patients. Here, AQP4 is linked to myelitis.