Thrombotic thrombocytopenic purpura (TTP) was originally described 70 years ago. It is considered an uncommon disorder with a reported occurrence rate of one case per 1 million patients. Mortality has decreased from almost 100% early on to 30-50% with the advent of newer treatment methods. We reviewed 41 patients with a diagnosis of TTP spanning the years 1980 to mid 1994. We found a much higher case rate, one per 6000 hosptial admissions, and an overall death rate of 40%. However, isolating 5 year periods we noted a marked fall in mortality from 54% (1980-1984), 44% (1985-1989), to 18% (1990-1994). Previous reports describe relapsing TTP and report an incidence of 7-15% although very recent data suggests a higher incidence. In our study, we found an overall relapse rate of 25% and by 5 year periods 23% (1980-1984), 13% (1985-1989), and 46% (1990-1994). We suggest that the improvement in survival and the increase in relapse rate are related and reflect more effective therapy for this once almost always fatal disease. Patients now survive their initial episode and thus are at risk for recurrence. Identification of risk factors for relapse will require further study.