Study objectives: Posttraumatic nightmares may exacerbate and perpetuate the daytime symptoms of posttraumatic stress disorder and might represent a therapeutic target. The therapeutic strategy of memory reconsolidation using the β-adrenergic receptor blocker propranolol associated with re-exposure psychotherapy is a promising treatment in patients with posttraumatic stress disorder. Previous studies have established this therapy is effective in reducing overall clinician-assessed posttraumatic stress disorder symptoms, but to date no previous study has specifically focused on posttraumatic nightmares in this therapy. This study provides a preliminary assessment of the evolution of nightmare severity during this therapy protocol compared with the decrease of the other posttraumatic stress disorder symptoms. It evaluates the incidence of side effects and examines the relative effects on posttraumatic nightmares. Methods: Patients were recruited as part of the Paris Mémoire Vive Study. Data were collected using a prospective longitudinal design including 1 baseline visit, 6 therapeutic visits, and 2 follow-up visits. During the 6 therapeutic visits, propranolol was administered orally 60 to 75 minutes prior to the psychotherapeutic session. Results: On average, nightmare severity decreased from "severe" to "mild" during the protocol and remained stable 2 months after the last session. Whereas 85% of patients reported nightmares at baseline, only 50% still had them after the protocol. The protocol was generally well tolerated and did not increase nightmare severity for any patient in the study. Conclusions: Memory reconsolidation therapy with propranolol seems promising in reducing nightmare severity, up to and including remission. However, research using a randomized controlled design and assessing maintenance of nightmare extinction is warranted. Clinical trial registration: Registry: ClinicalTrials.gov; Name: Using Reconsolidation Blockade to Treat Trauma Related Disorders After Paris Attacks: An Effectiveness Study (PARIS-MEM); Identifier: NCT02789982; URL: https://www.clinicaltrials.gov/ct2/show/NCT02789982. Citation: Mallet C, Chick CF, Maatoug R, Fossati P, Brunet A, Millet B. Memory reconsolidation impairment using the β-adrenergic receptor blocker propranolol reduces nightmare severity in patients with posttraumatic stress disorder: a preliminary study. J Clin Sleep Med. 2022;18(7):1847-1855.