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GNE myopathy is an adult-onset progressive myopathy, resulting from mutations in GNE, the key enzyme of sialic acid synthesis. The pathomechanism of GNE myopathy likely involves aberrant sialylation, since administration of sialic acid itself, or its precursor, N-acetylmannosamine (ManNAc), rescued hyposialylation of GNE myopathy mice. Recently, clinical trials for GNE myopathy patients were initiated. A robust, noninvasive biomarker is highly desirable for diagnosis of GNE myopathy and for evaluating response to therapy. Since muscle biopsies of patients with GNE myopathy demonstrated hyposialylation of predominantly O-linked glycans, we analyzed the O-linked glycome of patients’ plasma proteins using mass spectrometry. Most patients showed increased plasma levels of the core 1 O-linked glycan, Thomsen-Friedenreich (T)-antigen and/or decreased amounts of its sialylated form, ST-antigen. In addition, compared to unaffected individuals, all analyzed patients had a consistently increased ratio of T-antigen to ST-antigen. Importantly, the T/ST ratios were in the normal range in a GNE myopathy patient treated with intravenous immunoglobulins as a source of sialic acid, indicating response to therapy. Natural history and clinical trial data will reveal whether T/ST ratios can be correlated to muscle function. These findings not only highlight plasma T/ST ratios as a robust blood-based biomarker for GNE myopathy, but may also help explain the pathology and course of the disease.