Thrombotic thrombocytopenic purpura (TTP) and systemic lupus erythematosus (SLE) are both multisystem diseases and the latter can manifest hemopoietic abnormalities that may mimic TTP. This has led to diagnostic confusion and reports of the 2 diseases occurring in a single patient. We describe a 15-yr-old girl who presented at age 12 with purpura, fever, headaches, changes in conscious state, thrombocytopenia and microangiopathic hemolytic anemia and who was diagnosed despite the absence of renal disease, as having TTP. ANA and LE cells were negative then, and again 1 yr later. Three yr later she presented with the nephrotic syndrome with a positive ANA (1:100), elevated DNA antibody (79 U/ml, normal less than 25) and a circulating anticoagulant. Renal biopsy confirmed the presence of lupus nephritis. The association is discussed.