<jats:sec><jats:title>Aims</jats:title><jats:p>Previous uncontrolled studies suggested a possible benefit of intravenous immunoglobulin (IVIg) in parvovirus B19 (B19V)‐related dilated cardiomyopathy (DCM). This randomized, double‐blind, placebo‐controlled, single‐centre trial investigated the benefits of IVIg beyond conventional therapy in idiopathic chronic DCM patients with B19V persistence.</jats:p></jats:sec><jats:sec><jats:title>Methods and results</jats:title><jats:p>Fifty patients (39 men; mean age 54 ± 11 years) with idiopathic chronic (>6 months) DCM on optimal medical therapy, left ventricular ejection fraction (LVEF) <45%, and endomyocardial biopsy (EMB) B19V load of >200 copies/µg DNA were blindly randomized to either IVIg (<jats:italic>n</jats:italic> = 26, 2 g/kg over 4 days) or placebo (<jats:italic>n</jats:italic> = 24). The primary outcome was change in LVEF at 6 months after randomization. Secondary outcomes were change in functional capacity assessed by 6‐min walk test (6MWT), quality of life [Minnesota Living with Heart Failure Questionnaire (MLHFQ)], left ventricular end‐diastolic volume (LVEDV), and EMB B19V load at 6 months after randomization. LVEF significantly improved in both IVIg and placebo groups (absolute mean increase 5 ± 9%, <jats:italic>P</jats:italic> = 0.011 and 6 ± 10%, <jats:italic>P</jats:italic> = 0.008, respectively), without a significant difference between groups (<jats:italic>P</jats:italic> = 0.609). Additionally, change in 6MWT [median (interquartile range) IVIg 36 (13;82) vs. placebo 32 (5;80) m; <jats:italic>P</jats:italic> = 0.573], MLHFQ [IVIg 0 (−7;5) vs. placebo −2 (−6;6), <jats:italic>P</jats:italic> = 0.904] and LVEDV (IVIg −16 ± 49 mL/m<jats:sup>2</jats:sup> vs. placebo −29 ± 40 mL/m<jats:sup>2</jats:sup>; <jats:italic>P</jats:italic> = 0.334) did not significantly differ between groups. Moreover, despite increased circulating B19V antibodies upon IVIg administration, reduction in cardiac B19V did not significantly differ between groups.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>Intravenous immunoglobulin therapy does not significantly improve cardiac systolic function or functional capacity beyond standard medical therapy in patients with idiopathic chronic DCM and cardiac B19V persistence.</jats:p><jats:p>Clinical Trial Registration: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://ClinicalTrials.gov">ClinicalTrials.gov</jats:ext-link> ID NCT00892112.</jats:p></jats:sec>