Glucocorticoid (GC) refractory relapses in patients with multiple sclerosis (MS) or clinically isolated syndrome (CIS), who are in potential need of treatment escalation, are a key challenge in routine clinical practice. The pro-inflammatory cytokine macrophage migration inhibitory factor (MIF) has been shown to be an endogenous counter-regulator of GC, and potentiates autoimmune-mediated neuroinflammation. In order to evaluate whether MIF levels are elevated in the cerebrospinal fluid (CSF) of MS patients (CSF-MIF), and whether they are higher still during a GC refractory relapse, we compared CSF-MIF concentrations of CIS/MS patients with acute optic neuritis as their first inflammatory episode (ON, n = 20), CIS/MS patients with a stable disease progression/without relapse (CIS/MS w/o, n = 18), and healthy controls (HC, n = 20) using ANOVA. Mean CSF-MIF concentrations in CIS/MS w/o patients were significantly higher than in ON patients and HCs, whereas ON patients and HCs did not differ. A subgroup analysis of the ON group revealed 10 patients to be responsive to GC-treatment (GC-ON) and 10 patients refractory under GC-treatment (rGC-ON). However, mean CSF-MIF concentrations did not differ between GC-ON and rGC-ON cases. We therefore conclude that MIF is not suitable for distinguishing GC responders from non-responders in a group of patients with acute optic neuritis, but it rather mirrors the ongoing inflammation in long-term MS disease progression.