Optic neuritis (ON) patients can be divided into two groups based on the presence or absence of asymptomatic demyelinating lesions (ADLs) on brain MRI. The presence of ADLs is associated with an increased risk of progression to clinically definite multiple sclerosis (CDMS). The clinical data and brain MRI of 110 patients with acute unilateral ON were analyzed. Patients with ADLs had a significantly higher incidence of ON in spring months as compared with patients with no ADLs (p = 0.0024). Increased incidence of ON in spring months was seen in patients with ADLs whether or not they were diagnosed with CDMS on follow-up.