The study was undertaken to investigate clinical characteristics of thrombotic thrombocytopenic purpura (TTP) in patients with SLE and to determine risk factors and clinical outcome of TTP in patients with SLE. Among the 1203 patients with SLE admitted to catholic medical centre of the catholic university of Korea from January 1990 to December 2006, 26 patients with SLE were found to admit with TTP. TTP was defined if microangiopathic haemolytic anaemia, thrombocytopenia and negative Coombs' test were present and when at least one of the following signs was noted: renal impairment, neurologic deficit or fever. Eighty-seven patients with SLE who admitted with other manifestations, matched for age and sex, were included as disease controls. Data were retrospectively analysed based on medical records. There were no significant demographic characteristics between SLE patients with TTP and those with other manifestations. Multivariate analysis showed that independent risk factors for the development of TTP included high SLE disease activity index score (SLEDAI > 10, P = 0.006) and coexisting nephritis (P = 0.004). Among the 26 SLE patients with TTP, 12 died during admission period (in-hospital mortality rate: 46.1%). SLE patients with infection or neurologic manifestations had higher mortality rates. Multivariate analysis showed that infection is the only independent risk factor for mortality in SLE patients with TTP (P = 0.035). Patients with SLE who are in the active stage or who have renal involvement have the increased risk for TTP. Development of TTP in patients with SLE can be fatal. Therefore, intensive therapy will be needed especially in the presence of infection.