Background and Objective. To investigate the relationship between optic nerve sheath diameter, optic nerve diameter, visual acuity and osteopontin, and neurofilament heavy chain in patients with acute optic neuritis. Patients and Methods. Sonographic and visual acuity assessment and biomarker measurements were executed in 23 patients with unilateral optic neuritis and in 19 sex-and age-matched healthy controls. Results. ONSD was thicker on the affected side at symptom onset (median 6.3 mm; interquartile range 6.0-6.5) than after 12 months (5.3 mm; 4.9-5.6; p < 0 001) or than in controls (5.2 mm; 4.8-5.5; p < 0 001). OND was significantly increased in the affected side (3.4 mm; 2.9-3.8) compared to healthy controls (2.7 mm; 2.5-2.9; p < 0 001) and was thicker at baseline than after 12 months (2.8 mm; 2.7-3.0; p < 0 01). Visual acuity improved significantly after 12 months (1.00; 0.90-1.00) compared to onset of symptoms (0.80; 0.40-1.00; p < 0 001). OPN levels were significantly higher in patients at presentation (median 6.44 ng/ml; 2.05-10.06) compared to healthy controls (3.21 ng/ml, 1.34-4.34; p < 0 03). Concentrations of NfH were significantly higher in patients than in controls. Conclusion. ONSD and OND are increased in the affected eye. OPN and NfH are elevated in patients, confirming the presence of any underlying inflammation and axonal injury.