Reported cases of HUS/TTP (hemolytic uremic syndrome/thrombotic thrombopenic purpura) with cyclosporin (CsA) are more and more frequent, which led us to evaluate the incidence of microangiopathic process in kidney graft patients. In our first retrospective study we noted 9.4 percent of HUS/TTP among 117 patients. We tried to detect infraclinical hemolytic events prospectively in 40 new patients by systematic measurement of haptoglobin levels after introduction of CsA. The incidence of falls in haptoglobin levels was 25 percent. The immunological context was always noted: rejection crisis in 5 cases, cytomegalovirus infection in 3 cases, isolated markers of lymphocyte-activity in 2 cases. We never found a clinical or infraclinical form of microangiopathic process without a symptomatic or asymptomatic immunological complication. Monitoring haptoglobin levels make it possible to detect early endothelial injuries which could be linked to the future arteriolopathy described with CsA therapy.