Fatigue is a prominent feature of systemic lupus erythematosus (SLE), usually ascribed to various factors, such as muscle or joint involvement, anaemia or depression. The Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoantibody-mediated disorder of neuro-muscular transmission. We report on a well-defined LEMS associated with thymus hyperplasia in a SLE patient. An African 41 years-old SLE patient presented with persisting fatigue, myalgia and dyspnea, abolished reflexes and a bilateral ptosis. Neuromuscular electrodiagnostic study showed a clear-cut potentiation that was typical of a pre-synaptic neuromuscular junction disease. Anti-calcium gated channels antibodies were disclosed in serum and a diagnosis of LEMS was made. A total body CT-scan revealed an antero-superior mediastinal mass, compatible with thymoma. The tumour was surgically removed with a final diagnosis of follicular thymic hyperplasia. In conclusion, our observation provides a new example of entangled organ-specific and systemic autoimmunity in the context of thymus pathology. Potentiation study during electromyography should be performed systematically to rule out LEMS in patients with SLE and muscle weakness.