A 55-year-old woman with a several-decade history of thyroid goiter is presented here as a case of myasthenia gravis complicated with hyperthyroidism and thymoma with serological evidence of systemic lupus erythematous (SLE). She had had right eyelid ptosis since July 1992, with a positive tensilon test. The acetylcholine receptor antibody titer was 4.01 nmol/L. A thyroid function test revealed T3: 162 ngidl, T4: 14.98 micrograms/dl, TSH:0.09 microIU/ml and positive anti-microsomal antibody (1:400). An MRI of the chest showed a thymoma in the left thymus. Other autoantibody screenings include ANA (1:320, speckled pattern) and anti-ds DNA (+) suggesting a serological association with SLE. After three courses of plasmapheresis, she received an extended maximal thymomectomy and a subtotal thyroidectomy. She was then treated with prednisolone, Mestinon, Eltroxin and discharged without complications. The coexistence of myasthenia gravis, hyperthyroidism, thymoma and a serological evidence of SLE have not previously been documented in the literature.