Central nervous system (CNS) relapse in patients with diffuse large B-cell lymphoma (DLBCL) carries a dismal prognosis and most clinical guidelines recommend CNS prophylaxis to patients deemed at high risk of CNS relapse. However, results from observational studies investigating the effect of CNS prophylaxis have yielded conflicting results. The aims of this study were to evaluate: (i) whether addition of prophylactic intravenous high-dose methotrexate (HD-MTX) reduces the risk of CNS relapse in high-risk DLBCL patients treated with R-CHOP or similar, and (ii) whether HD-MTX prophylaxis confers an overall survival benefit, irrespective of CNS relapse. We performed a systematic search of MEDLINE/PubMed and EMBASE for data on DLBCL patients at high risk of CNS relapse treated with R-CHOP or similar who received HD-MTX as an intervention and a comparator arm of patients who did not receive prophylaxis and/or intrathecal prophylaxis. A risk of bias was estimated using the ROBINS-I tool and the quality of the evidence was assessed by the GRADE approach. Finally, a meta- analysis based on the systematic review was conducted. A total of 1,812 studies were screened. No randomized controlled trials were identified. Seven observational studies comprising 1,661 patients met the inclusion criteria. We found a statistically non-significant relative risk of 0.54 (95% confidence interval: 0.27-1.07) of CNS relapse for patients receiving HD-MTX versus controls. The meta-analysis investigating mortality demonstrated a relative risk of death of 0.70 (95% confidence interval: 0.44-1.11) for patients treated with HD-MTX versus controls. The overall risk of bias was adjudged as "serious" and the quality of the evidence was rated as "low". In conclusion, our data indicate that HD-MTX does not prevent or, at best, only slightly reduces the risk of CNS relapse and confers no survival benefit.