The strengths and limitations of our study are as follows: this is the first study to examine the relationship between APOE serum levels and APOE ε2, ε3, and ε4 alleles with ON and MS in the Baltic states, and this is the first study to divide patients into two groups, namely, patients that only had isolated optic neuritis and patients that had optic neuritis and multiple sclerosis. Here, APOE is linked to multiple sclerosis.