BACKGROUND Cerebrospinal fluid (CSF) may provide markers of severity and outcome of optic neuritis. METHODS We examined the CSF from 29 patients with acute monosymptomatic optic neuritis (AMON) for content of myelin basic protein-like material (MBPLM) and correlated the levels with results of cranial magnetic resonance imaging. The length of the time from onset of AMON to the time of CSF collection did not exceed four weeks. RESULTS Only two patients (7%), one of whom developed an acute myelopathy one month after AMON, showed an elevated value of CSF MBPLM. No MBPLM was detected in 6 patients (21%), and other 21 (72%) had detectable levels of MBPLM but below the upper limit of normal of 0.1 ng/ml. The value of MBPLM was not significantly correlated with the interval to CSF sampling from onset of AMON or with severity of decreased visual acuity. The highest values of CSF MBLPM were observed among patients with severely decreased visual acuity and among patients with an abnormal MRI (13 of 27 i.e. 48%). CONCLUSIONS CSF MBPLM was rarely abnormal in AMON. However, CSF MBPLM may have potential value in reflecting disease activity, as the highest values were obtained among patients with CSF sampled soon after the maximum visual dysfunction was reached, with severe visual impairment, and with an abnormal MRI.