The sonidegib and ruxolitinib combination was assessed in an open-label study in JAK inhibitor-naive patients with myelofibrosis (MF). The primary objective of phase 1b was to establish the maximum tolerated dose (MTD) and/or recommended phase 2 dose (RP2D) and phase 2 was to assess spleen volume reduction at weeks 24 and 48. Fifty patients were enrolled. In the dose-escalation phase (n = 23), doses for sonidegib once daily/ruxolitinib twice daily were 400/10 mg (level 1, n = 8), 400/15 mg (level 2, n = 10), and 400/20 mg (level 3, n = 5). Two patients had dose-limiting toxicity at level 2: increased blood creatine phosphokinase (grades 3 and 4, n = 1 each). MTD/RP2D was determined as sonidegib 400 mg daily + ruxolitinib 20 mg twice daily. In phase 1b expansion and phase 2 stage 1 (n = 27), by weeks 24 and 48, ≥35% reduction in spleen volume was observed in 44.4% and 29.6% patients, respectively. By weeks 24 and 48, 42.0% and 26.0% patients had ≥50% reduction in Myelofibrosis Symptom Assessment Form total symptom score, respectively. Most common treatment-related adverse events (grade 3/4) were increased blood creatine phosphokinase (18%), anemia (14%), and thrombocytopenia (12%). Four deaths were reported due to multiple organ dysfunction syndrome (on-treatment; no relationship with study treatment), acute myeloid leukemia, MF progression, and aspiration pneumonia. Although well tolerated, this combination will not be further developed in MF patients due to modest overall benefit compared with historical ruxolitinib monotherapy. This trial was registered at www.clinicaltrials.gov as #NCT01787552.© 2020 by The American Society of Hematology.