Background: Patients with Hailey-Hailey and Darier diseases present with disabling inflammatory lesions located in large skin folds, which are often exacerbated or induced by sweating. Quality of life is highly impaired because of pain and recurrent skin infections. An improvement in skin lesions after botulinum toxin A injections has previously been reported in some patients but no prospective interventional studies are available. The aim of this open-label, 6-month, interventional pilot study (NCT02782702) was to evaluate the effectiveness and safety of botulinum toxin A injections. Thirty patients with moderate to very severe skin lesions located in folds were enrolled in the study. Results: Thirty patients (26 Hailey-Hailey / 4 Darier) were included. Botulinum toxin A proved effective within the first month in two-thirds of patients, taking all study parameters (itchiness, cutaneous pain, sweating and odour, infections, psychosocial impairment and quality of life) into account and persisted during the 6-month follow-up period. No patient was classed as a BtxA non-responder, but 11 (37%) patients, who were the most severe ones, experienced a relapse during the study. No serious side effects were reported. Mild transient clear fluid discharge at the site of the injections was reported for 27% of patients. Conclusions: Botulinic toxin is an effective and safe treatment for Hailey-Hailey and Darier diseases. Further studies should evaluate the benefit of increased doses or combined treatments for the most severe patients.Timing of approvals:- 25, February 2015: approval by the institutional ethics committee- 2, June 2015: registration under the European Eudract identifier 2015-000379-29 (https://eudract.ema.europa.eu/index.html)- 31, July 2015: approval by the French national health authority, ANSM - 31, August 2015: implementation agreement signed by the sponsor. - 25, May 2016: registration under the ClinicalTrials.gov identifier NCT02782702 (https://clinicaltrials.gov/ct2/show/NCT02782702?cond=NCT02782702&draw=2&rank=1).