These categories describe the presentations of patients in The Oklahoma TTP-HUS Registry for whom plasma exchange treatment was begun for clinically suspected TTP-HUS.7 The approximate relative frequencies of these categories are: idiopathic, 40%; additional/alternative explanation, 20%; others, 10%. Not included among these categories are children, most of whom present with bloody diarrhea11 and who are not treated by plasma exchange; the youngest patient in our Registry is 14 years old. All patients fulfilled diagnostic criteria for TTP-HUS. The assignment of drug-induced categories as allergic or dose-related toxicity is based on clinical observations. Among patients with an additional or alternative explanation for the clinical features, some had an apparent additional diagnosis (eg, systemic lupus erythematosus44) when plasma exchange treatment was begun. In others an alternative explanation for the clinical features only became apparent later, and then treatment was stopped (see Table 3). Selected references are provided.