SummaryIdiopathic inflammatory myopathies (IIM) are a group of chronic autoimmune diseases mainly affecting proximal muscles. Absence of meaningful prognostic factors in IIM has hindered new therapies development. Glycans are essential molecules that regulate immunological tolerance and consequently the onset of autoreactive immune response.We showed that muscle biopsies from patients with IIM revealed a deficiency in the glycosylation pathway resulting in loss of branchedN-glycans. At diagnosis, this glycosignature predicted disease relapse and treatment refractoriness. Peripheral CD4+T cells from active-disease patients shown a deficiency in branchedN-glycans, linked to increased IL-6 production. Glycan supplementation, restoring homeostatic glycosylation profile, led to a decrease in IL-6 levels.This study highlights the biological and clinical importance of glycosylation in IIM immunopathogenesis, providing a potential mechanism for IL-6 production. This pinpoints muscle glycome as promising biomarker for personalized follow-up and a potential target for new therapies in a patients’ subgroup with an ominous evolution.