SummaryNine years after thymectomy, efficient on the neurologic symptoms of Myasthenia Gravis, a 29 years old female developped systemic Lupus Erythematosus. There was no exacerbation of Myasthenia Gravis and immunological investigations showed a T lymphocyte defect. This is the fourth reported case of systemic Lupus Erythematosus after adult thymectomy for Myasthenia Gravis. The influence of adult thymectomy on the immune competence is reviewed. The possible role of this operation on the appearance of the lupus syndrome is discussed.