Introduction: Chemical-mechanical preparation enables antisepsis of the root canal system through the action of chemical substances, being crucial for a favorable prognosis of endodontic treatment. In 2010, two new systems were introduced to the market, aiming at instrumentation by means of the single-file reciprocating technique. Objective: To report a clinical case of endodontic intervention carried out by means of Reciproc R25 instrument. Case report: A Caucasian female patient was referred for endodontic evaluation. Clinical examination showed Class I composite resin restoration and fixed, supported prosthesis between teeth #44 and #45, active fistula, absence of both periodontal pocket and dental mobility. Thermal and electrical pulp sensitivity tests yielded negative results. Radiographic examination revealed both double-curved root canal and diffuse rarefying osteitis. Diagnosis of pulp necrosis and chronic periapical abscess of tooth #44 was established, and endodontic therapy was then instituted. In the first visit, the root canal was biomechanically prepared by means of asymmetric reciprocation with Reciproc R25 files, intracanal dressing with Ca(OH) 2 PA and propylene glycol, and double coronal sealing with coltosol and glass ionomer for 30 days. In the second visit, the root canal system was filled. The patient has been asymptomatic for two years, exhibiting clinical and radiographic evaluations within normality. Conclusions: The Reciproc system contributed decisively to chemical-mechanical preparation, thereby contributing to the success of the proposed clinical case. How to cite this article: Luz Segundo ACS, Silva RV, Pereira RP, Nunes E. Use of Reciproc in mandibular premolar with double-curved root canals: a case report.