Two cases of invasive thymoma in patients with systemic lupus erythematosus (SLE) are described. In both instances the suspected diagnosis of a mediastinal tumour proved difficult to confirm. Neither surgical removal of the thymoma in one case nor irradiation in the other had any apparent clinical or serological effect on the course of the SLE. Both patients subsequently suffered from respiratory problems and the distinction between recurrent tumour and pulmonary involvement by SLE proved difficult. One patient had a good response, however, to pulse methylprednisolone, but the other later developed recurrence of tumour and died of Pneumocystis carinii infection following cytotoxic therapy three years after discovery of the tumour.