BACKGROUND: Presence of oligoclonal bands (OCBs) restricted to cerebrospinal fluid (CSF) characterizes most patients with multiple sclerosis (MS). Few data are available on the frequency of MS diagnosis and the main alternative diagnoses in patients with an initial central nervous system (CNS) demyelinating event and CSF IV pattern, the so-called 'mirror pattern'. METHODS: Seventy-six patients presenting with OCBs pattern IV after a clinical attack suggestive of CNS demyelinating event were included in the study. Diagnostic work-up, including blood, CSF, and paraclinical examinations, and 2 years of clinical and radiological follow-up were evaluated. RESULTS: Pattern IV occurred in 15.1% of patients. Twenty-five patients (32.8%) received a diagnosis of MS, thirty-two (42.1%) an alternative diagnosis, and nineteen (25%) remained without definite diagnosis. Most frequent alternative diagnosis was encephalopathy with atypical MRI lesions of probable vascular origin (19.7%). MS was significantly more common in patients with type IV OCB pattern (25 of 76) than in a group of patients presenting with type I OCB pattern (32 of 168, p = 0.017). CONCLUSION: The diagnosis of MS is common in patients who present with OCBs pattern IV. However, other CNS disorders, particularly vascular encephalopathy, should be carefully considered.