A measure of the size of the thymus in 24 patients with systemic lupus erythematosus was obtained from the lateral cross-sectional area determined roentgenologically by anterior mediastinography and tomography; the mean cross-sectional area of the thymus was 4.1 ± standard deviation 2.3 sq cm which differed significantly from values for controls. A ratio of 2.5 was derived to convert the cross-sectional area in square centimeters to an "estimated weight" in grams. The thymus in patients with established systemic lupus erythematosus (SLE) was small, the estimated weight in many being less than 10 gm whether or not corticosteroids had been given. Five patients were examined before and three months after beginning treatment with prednisolone. In patients with SLE of acute onset an appreciable reduction in size was demonstrated in contrast to patients with long-standing disease in whom thymic size was unchanged.