The case described concerns a patient initially presenting the clinical symptoms of rheumatoid arthritis, including the presence of nodules, and slowly developing into systemic lupus erythematosus (SLE). Besides the signs of chronic inflammation in the synovial tissue, many granulocytes and PAS-positive macrophages were present. At electronmicroscopy the basal membranes of the vessels were multilaminated and the endothelial cells enlarged while many macrophages contained large clumps of electrondense material which could easily be interpreted as rough endoplasmatic reticulum (RER) of plasma cells. Immunoflourescence studies showed deposition of immunoglobulins and complement in the vessel walls of the synovium, thus suggesting an immune-complex pathogenesis.