Background: Some environmental factors have been already associated to increased risk of multiple sclerosis (MS), but it is plausible that additional factors might play a role. Objective: To investigate in MS patients the relationship between inflammatory activity, detected by brain magnetic resonance imaging (MRI) with gadolinium (Gd), and air pollution, namely, particulate matters with diameter less than 10 μm (PM₁₀). Methods: We analyzed from 52 remitting MS patients 226 brain MRIs, 34% with (Gd+MRI) and 66% without (Gd-MRI) T1-Gd-enhancing lesions. Daily recording of PM₁₀ in the 30 days before MRI examination was obtained by monitors depending on the residence of subjects. Results: PM₁₀ levels in the 5, 10, 15, 20, and 25 days before brain MRIs were higher (plus 16%, 21%, 24%, 25%, and 21%, respectively) with reference to Gd+MRI versus Gd-MRI. There was a significant association between Gd+MRI and PM₁₀ levels ( p = 0.013), independent of immune therapies, smoker status, and season. In patients who had two repeated MRIs with opposite outcomes (Gd-MRI and Gd+MRI), PM₁₀ levels were strongly higher in concurrence with Gd+MRI ( p < 0.0001). Conclusion: Our findings suggest that air pollution may be a risk factor for MS favoring inflammatory exacerbations.