Background <p>Chronic inflammatory cardiomyopathy (infl-CMP) is a long-term sequela caused by the chronicity of acute myocarditis, especially fulminant myocarditis (FM). Hydroxychloroquine (HCQ) may benefit these patients by inhibiting the excessive inflammatory response.</p> Methods <p>In this multicenter, randomized trial, we evaluated the efficacy and safety of HCQ in patients with chronic infl-CMP after FM. The primary outcome of the trial was a composite of the cardiovascular outcomes of time to cardiovascular death or heart transplant, hospitalization for heart failure or recurrence of myocarditis, permanent pacemaker, or implantable cardioverter defibrillator implantation. Secondary outcomes were changes in left ventricular ejection fraction (LVEF), left ventricular internal diastolic diameter (LVIDd), plasma levels of high-sensitivity cardiac troponin I (hs-cTnI), N-terminal pro-B-type natriuretic peptide (NT-proBNP), high-sensitivity C-reactive protein (hs-CRP), and erythrocyte sedimentation rate (ESR) from baseline to 12&#xa0;months.</p> Results <p>Fifty patients were randomized to receive HCQ combined with prednisolone (PDN) or PDN monotherapy for 12 months. Compared to PDN monotherapy, HCQ combined with PDN therapy reduced the primary composite outcome [hazard ratio (HR) = 0.28, 95% confidence interval (CI) = 0.11–0.71] and had significant changes in the increase of LVEF and the decrease of LVIDd, hs-cTnI, NT-proBNP, and hs-CRP in patients with infl-CMP. No serious drug-related adverse events were recorded in either group, indicating an acceptable safety profile. Furthermore, HCQ combined with PDN significantly reduced the levels of 16 plasma cytokines to levels comparable to healthy controls.</p> Conclusions <p>Twelve months of HCQ combined with PDN therapy significantly improved the prognosis and heart function, inhibited inflammation, and had acceptable safety in patients with infl-CMP after FM.</p> Trial registration <p>ClinicalTrials.gov identifier: NCT05961202.</p> Graphical Abstract <p></p>