Modern immunomodulatory treatment of MS calls for a better recognition and early diagnosis of MS patiens with uncommon cerebrospinal fluid (CSF) findings. We analyzed clinical features and CSF findings of OT-negative MS andMS with >= 15 mononuclear cells/mm3 comparing OT-negative and OTpositive MSpatients and patients with >= 15 CSF mononuclear cells/mm3 and 15 CSF mononuclear cells/mm3, respectively. Retrospective analysis included 233 patients with definite MS. Twenty-six patients had OT-negative MS, 26 patientshad MS with = 15 mononuclear cells/mm3 in CSF. The diagnosis was delayed in OT-negative MS, the average time to diagnosis being 9.21 years (SD 11.24) compared to 4.59 years (SD 5.53) in OT positive MS (p=0.01). The concentration of CSF mononuclear cells was lower, 4.81/ mm3 (SD 4.21) comparedto 7.74/ mm3 (SD 7.57); p=0.05, as was IgG level, 9.62% (SD 5.52) compared to 17.19% (SD 9.93) in OT-positive MS p=0.0001. The patients with increased CSF mononuclear cell concentration were diagnosed earlier, the average time to diagnosis being 2.31 years (SD 2.95) compared to 5.46 years (SD 6.84); p=0.02. The patients were more often females and had relapsing-remitting MS (the observation is not statistically significant). They had elevated IgG level, 20.31 % (SD 11.03) compared to 15.18% (SD 9.45); p=0.01. OT-negative MS and MS with increased concentration of CSF mononuclear cells are both found in approximately 10% patients with definite MS. The diagnosis is delayed, because of absence of OT and less active CSF. On the other hand, MS with increased mononuclear cell concentration manifests with more active CSF, what allows an earlier diagnosis. The patients have more often relapsing-remitting MS, which is the form more suitable for introductionof immunomodulatory treatment.