IntroductionRecurrent miscarriage (RM), defined by three or more consecutive losses during the first trimester of pregnancy, affects 1%–2% of fertile couples. Standard investigations fail to reveal any apparent cause in ~50% of couples. However, on the basis of animal models and clinical studies, several hypotheses have been put forward concerning underlying mechanisms of RM: altered ovarian reserve, progesterone defect, thrombotic and/or endothelial dysfunction and immunological disturbances. Nonetheless, no study has yet reached conclusive beneficial clinical evidence for a potential treatment in unexplained RM. Hydroxychloroquine (HCQ) is a molecule with extensive safety data during pregnancy. The pharmacological properties of HCQ (eg, antithrombotic, vascular protective, immunomodulatory, improved glucose tolerance, lipidlowering and anti-infectious) could be effective against some mechanisms of unexplained RM. Furthermore, eventhough clinical benefit of HCQ is suggested in prevention of thrombotic and late obstetric events in antiphospholipid (APL) syndrome, there are no data suggesting the benefit of HCQ in RM in the presence of APL antibodies.Methods and analysisTaken all together and given the low cost of HCQ, the aim of this multicentre, randomised, placebo-controlled, double-blind study is to investigate whether HCQ would improve the live birth rate in women with RM, irrespective of maternal thrombophilic status: (1) no known thrombophilia, (2) inherited thrombophilia or (3) APL antibodies. The primary end point is a live and viable birth. After confirming eligibility and obtaining consent, 300 non-pregnant women will be randomised into two parallel groups for a daily oral treatment (HCQ 400 mg or placebo), initiated before conception and stopped at 10 weeks’ gestation. If pregnancy does not occur after 1 year, the treatment will be stopped.Ethics and disseminationAgreement from the French National Public Health and Drug Security Agency (160765A-22) and ethical approval from the Committee for the Protection of Persons of NORD-OUEST I (2016-001330-97) have been obtained.Trial registration numbersNCT0316513; Pre-results.