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Background Serum neurofilament light chain (sNfL) is an emerging biomarker that correlates with inflammatory activity and treatment response in multiple sclerosis (MS). We sought to determine the utility of a single sNfL in predicting disease activity in patients treated with ofatumumab and ocrelizumab, and to compare this to patients on no/low/medium efficacy disease modifying therapy (DMT).Methods Patients with a diagnosis of relapsing-remitting MS and treated with B cell therapy or no/low/medium efficacy DMTs had a single sNfL sampled at recruitment. Three parameter ‘no evidence of disease activity’ (NEDA-3) criteria were assessed every 6–12 months.Results 195 patients treated with ocrelizumab or ofatumumab, with a mean age of 49.1 (±12.4) years, were followed up for a median duration of 2.6 years from sNfL sampling. Median EDSS remained stable during the study period (median EDSS 1.5 [IQR 1–3.5] to 1.5 [1–4]). 15.9% had evidence of disease activity (EDA) after a minimum of six months on B cell therapy. The difference in sNfL between NEDA-3 and EDA patients was statistically significant (p<0.0001). Receiver operating characteristic analysis found that a single sNfL had a fair ability to predict EDA. B cell treated patients had significantly lower sNfL values than those on no/low/medium efficacy therapy (p<0.0001).Conclusion In patients on B cell therapy, a single sNfL result has reasonable ability in predicting EDA and raw values are significantly lower in those meeting NEDA-3 criteria. These findings further validate the utility of sNfL in the real-world setting.