Hedgehog signaling plays a key role in tissue fibrosis, the pathological hallmark of chronic graft-versus-host disease (cGVHD). We conducted a phase 1 trial of sonidegib, a selective antagonist of the hedgehog coreceptor Smoothened, for the treatment of steroid-refractory cGVHD. After a 3+3 study design, sonidegib was administered for up to 12 cycles of 28 days each, using 3 doses: 200 mg/day (dose level 1), 400 mg/day (dose level 2), and 600 mg/day (dose level 3). Seventeen patients were enrolled. The median number of cycles completed was 6 (range, 0-12). There was only 1 dose-limiting toxicity (cohort 2, grade 3 creatine phosphokinase increase) observed. Immunohistochemical evaluation of skin biopsies revealed decreased protein expression of hedgehog signaling pathway molecules with sonidegib therapy. Clinically, 8 patients (47%) had a partial response in skin or sclerodermatous disease, 6 patients had no response, and 3 were not evaluable. Clinical responses were assessed by treating physicians and not by National Institutes of Health criteria. Overall, patients reported worsening of quality of life, which was more severe in clinical nonresponders. Accrual was terminated early as a result of the cumulative toxicity burden not attributed to sonidegib and patient decisions to stop taking sonidegib. We believe hedgehog signaling inhibition warrants further investigation in patients with cGVHD because of the association with clinical responses and immunohistochemical changes. This trial was registered at www.clinicaltrials.gov as #NCT02086513.