Background: In vitro studies of BTK and PI3K inhibitors demonstrate synergy in non-Hodgkin lymphoma (NHL). We embarked on a phase I/Ib investigator-initiated clinical trial evaluating the combination of ibrutinib (BTK inhibitor) and buparlisib (pan-PI3K inhibitor) in relapsed/refractory (R/R) NHL. The completed dose escalation is reported. Methods: Patients (pts) were eligible if they had R/R DLBCL, MCL, or FL with ECOG≤2 and adequate organ function. Ibrutinib and buparlisib were given daily by mouth on a 28-day cycle. Dose reductions were permitted after cycle 1. Tumor response was based on Lugano Classification however CR required both PET resolution and ≥ PR by CT. Results: As of Dec 16, 2016, 13 pts were enrolled and evaluated for toxicity (DLBCL 5, FL 2, MCL 6). Dose levels and DLT per table. Six pts discontinued treatment for disease progression (DLBCL 4, FL 2). Hematologic AE ≥ grade 3 are anemia (2), leukocytosis (2), and leukopenia (4). Relevant non-hematologic AEs of any grade ≥ 20% across all pts were fatigue (77%), diarrhea (62%), anorexia (54%), rash (46%), hyperbilirubinemia (46%), gastric reflux (46%), CMV reactivation (31%), mood change (31%), and hypertension (23%). Most common related grade 3/4 toxicity is rash (N = 3). No grade 5 toxicities noted. Serious adverse events (SAE) include: grade 2 pleural effusion and grade 2 nausea (N = 1), grade 1 fever with hospitalization (N = 1), grade 2 confusion and grade 4 hyponatremia (N = 1) were unrelated to therapy. Responses noted in 13 pts: MCL (N = 6: CR 4, PR 2), FL (N = 2: SD 2), DLBCL (N = 5: SD 1). One CR was a MCL pt with CR after 2 cycles on combination therapy and continues in remission on ibrutinib alone because of buparlisib toxicity. Conclusions: Combination of ibrutinib and buparlisib while generally well tolerated has predicted toxicities of both BTK and PI3K inhibitors. The recommended phase 2 dose is ibrutinib 560 mg and buparlisib 100 mg though dose reductions for tolerability may be needed for long term oral therapies. Promising efficacy is observed in MCL. Clinical trial information: NCT02756247