OBJECTIVE This is the final report of a phase III randomized study to evaluate whole-brain radiotherapy (WBRT) in primary therapy of primary CNS lymphoma (PCNSL) after a median follow-up of 81.2 months. METHODS Patients with newly diagnosed PCNSL were randomized to high-dose methotrexate (HDMTX)-based chemotherapy alone or followed by WBRT. We hypothesized that the omission of WBRT would not compromise overall survival (OS; primary endpoint), using a noninferiority design with a margin of 0.9. RESULTS In the per-protocol population (n = 320), WBRT nonsignificantly prolonged progression-free survival (PFS) (median 18.2 vs 11.9 months, hazard ratio [HR] 0.83 [95% confidence interval (CI) 0.65-1.06], p = 0.14) and significantly PFS from last HDMTX (25.5 vs 12.0 months, HR 0.65 [95% CI 0.5-0.83], p = 0.001), but without OS prolongation (35.6 vs 37.1 months, HR 1.03 [95% CI 0.79-1.35], p = 0.82). In the intent-to-treat population (n = 410), there was a prolongation by WBRT of both PFS (15.4 vs 9.9 months, HR 0.79 [95% CI 0.64-0.98], p = 0.034) and PFS from last HDMTX (19.4 vs 11.9 months, HR 0.72 [95% CI 0.58-0.89], p = 0.003), but not of OS (32.4 vs 36.1 months, HR 0.98 [95% CI 0.79-1.26], p = 0.98). CONCLUSION Although the statistical proof of noninferiority regarding OS was not given, our results suggest no worsening of OS without WBRT in primary therapy of PCNSL. CLASSIFICATION OF EVIDENCE This study provides Class II evidence that in PCNSL HDMTX-based chemotherapy followed by WBRT does not significantly increase survival compared to chemotherapy alone. The study lacked the precision to exclude an important survival benefit or harm from WBRT.