Background: PCNSL is an aggressive primary brain tumor with median progression free survival (PFS) after upfront methotrexate-based chemotherapy of 2-3 years. Outcome and treatment options are poor for recurrent/refractory (r/r) disease. Ibrutinib has shown promising clinical response in Mantle cell lymphoma, CLL, Marginal Zone, and Waldenström. This trial investigates Ibrutinib in patients with r/r PCNSL and SCNSL. Methods: Eligible patients had r/r PCNSL or SCNSL, age≥18, ECOG≤2, normal end-organ function, and unrestricted number of CNS directed prior therapies. In patients with SCNSL disease, systemic disease needed to be absent. Results: Twenty-five patients were enrolled (3 at 560 mg; 22 at 840 mg). Median age was 68 (range 21-85); 15 were women. Median ECOG was 1 (0: 2, 1: 15, 2: 8). 64% had PCNSL and 36% SCNSL; 68% had recurrent disease. Seventeen had parenchymal disease, 3 isolated cerebrospinal fluid (CSF) involvement and 5 both. Seven grade 4 adverse events were observed in 7 patients neutropenia (in 3 patients), lymphopenia (2), sepsis (1), and ALT elevation (1). Fourteen patients developed 20 grade 3 toxicities, including lymphopenia in 5 patients, hyperglycemia in 3, ALT elevation in 2, thrombocytopenia in 2, lung infection in 2, AST elevation in 1, neutropenia in 1, urinary tract infection in 1, colitis in 1, febrile neutropenia in 1 and fungal encephalitis in 1. The most common toxicities at any grade were hyperglycemia, thrombocytopenia and anemia of which most were grade 1/2. No grade 5 events have been observed. After a median follow-up of 414 days (range 289-674), 22/25 patients were evaluated for response (3 did not complete at least 15 days of drug treatment). Over all response was 68% (17/22; 77% (17/22) in patient that completed at least 15 days of drug treatment) with 10 CR, 7 PR, 2 SD and 3 PD as best response. The median PFS is 4.6 months (5.4 months in patients that completed at least 15 days of drug treatment; longest: 15.3 months). The median overall survival has not been reached. Conclusions: Patients with CNS lymphoma tolerate Ibrutinib with manageable adverse events. Clinical response was seen in 68% of CNS lymphoma patients. Clinical trial information: NCT02315326