Systemic lupus erythematosus (SLE) and myasthenia gravis (MG) are autoimmune diseases, whose association in the same patient is rarely reported. Both pathologies share the following characteristics: affect mainly young women; alternate exacerbation and remission periods; and have positive antinuclear antibody (ANA) test. This case report assesses possible diagnostic hypotheses for the clinical findings of eyelid ptosis and proximal muscle weakness in a female patient recently diagnosed with SLE, who evolved with associated MG.