The coexistence of systemic lupus erythematosus (SLE) and thymoma is rare. We describe 2 female patients with this combination. A 48-year-old woman presented with dyspnea due to a left pleural effusion. Her past history revealed over the previous 3 years the development of anemia, thrombocytopenia, alopecia, pericardial effusion and proteinuria. Four months prior to this hospitalization, the patient was first admitted due to purpura. At that time, laboratory tests revealed an elevated sedimentation rate, elevated titers of ANA and anti-DNA. Chest X-ray demonstrated a widened mediastinum, and upon operation an encapsulated thymoma was excised. Four months following the thymectomy, the patient is unresponsive despite high dose steroid therapy. Another patient, a 30-year-old woman, presented with SLE (cutaneous, arthritis, anemia, positive ANA and high titers of anti-DNA) and thymoma simultaneously. Six years after thymectomy the patient is in SLE remission. Thymectomy in mice prone to autoimmunity (NZB/W mice) has been shown to accelerate the autoimmune manifestations. Conversely, the opposite effect is seen in MRL/lpr mice. The immunological effect of adult thymectomy on the course of human SLE remains to be established, on a larger series of patients. It seems that the heterogenicity of human patients is exemplified by the contrasting effects of thymectomy for thymoma in SLE patients.