Objective: Monoclonal gammopathy have been related with numerous neurological disorders but the spectrum of associated myopathies remains poorly described. We report a new acquired myopathy associated with monoclonal gammopathy. Methods: Three patients were prospectively analysed. In addition to patients’ characteristics, electrophysiological data, muscle biopsy analysis and outcomes of patients were collected. Results: Three patients aged from 38 to 56 years were analysed. All suffered from a muscle weakness with a sub-acute onset and stiffness, in a context of severe weight loss. The muscle deficit mainly involved the proximal limbs and axial muscles. Creatine kinase level was increased [1400-2900 I.U/L] and electromyography revealed a myogenic pattern and spontaneous high frequency discharges. Muscle biopsies showed the association of vacuoles filled with glycogen and mild inflammation. There was no evidence for genetic glycogen metabolic disorder. IgGκ monoclonal gammopathy was identified in all cases. There was no sign of lymphoplasmocytic proliferation. All patients improved with a treatment combining corticosteroid, intravenous immunoglobulin and immunosuppressants, and dramatic improvement was observed in two patients. Conclusion: we reported a new monoclonal gammopathy associated muscle disease defined by a vacuolar myopathy characterized by axial and proximal muscle weakness with prominent stiffness and high frequency discharges on electromyography.