BACKGROUND: Patients with relapsed or refractory high-risk chronic lymphocytic leukaemia or mantle cell lymphoma often do not derive durable benefit from ibrutinib monotherapy. We hypothesised that dual B-cell receptor pathway blockade would be tolerable and efficacious. We investigated a next-generation phosphoinositide-3-kinase-δ inhibitor (PI3K-δi), umbralisib, plus a Bruton tyrosine kinase inhibitor (BTKi), ibrutinib, in relapsed or refractory chronic lymphocytic leukaemia and mantle cell lymphoma. METHODS: We did an investigator-initiated, multicentre, phase 1-1b study of patients from five sites in the USA (academic and community sites). Patients were 18 years and older with relapsed or refractory chronic lymphocytic leukaemia or mantle cell lymphoma, with an Eastern Cooperative Oncology Group performance status of 2 or less, and were given umbralisib orally once daily (400 mg, 600 mg, or 800 mg) and ibrutinib orally once daily (420 mg for chronic lymphocytic leukaemia or 560 mg for mantle cell lymphoma) until disease progression or unacceptable toxicity. The phase 1 dose-escalation cohorts for each histology escalated independently in a standard 3 × 3 design. The primary endpoints were intention-to-treat assessment of maximum-tolerated dose, safety, and dose-limiting toxicities. This trial is ongoing and is registered with ClinicalTrials.gov, number NCT02268851. FINDINGS: Between Dec 5, 2014, and March 7, 2018, we enrolled 44 patients, of which 42 were given at least one dose of study drug (chronic lymphocytic leukaemia, n=21; mantle cell lymphoma, n=21). Patients had a median age of 68 years (range 48-85) and had a median of two (IQR 1-3) previous therapies. No dose-limiting toxicities were observed and the maximum-tolerated dose of umbralisib was not reached. The recommended phase 2 dose of umbralisib when given in combination with ibrutinib was 800 mg once daily. The most frequent adverse events included diarrhoea (22 [52%] patients, 10% of whom had grade 3), infection (21 [50%], 17% grade 3-4), and transaminitis (ten [24%], 2% grade 3). Serious adverse events occurred in 12 (29%) patients and included lipase elevation, atrial fibrillation, hypophosphataemia, adrenal insufficiency, transaminitis, and infections. INTERPRETATION: Umbralisib plus ibrutinib is well tolerated and active in relapsed or refractory chronic lymphocytic leukaemia and mantle cell lymphoma, with a recommended phase 2 dose of umbralisib 800 mg once daily. To the best of our knowledge, these are the first clinical data on a BTKi and PI3K-δi doublet in B-cell malignancies, and the results suggest that this approach is feasible and worthy of further study. FUNDING: TG Therapeutics, Leukemia and Lymphoma Society Therapy Accelerator Program.