Most cases of thrombotic thrombocytopenic purpura (TTP) are considered idiopathic without an identifiable etiologic agent. It has been previously reported that a number of patients with TTP had a urinary tract infection (UTI). Apheresis records were searched for patients with TTP from 1999 through 2007. Records were examined for evidence of UTI, and the patients were divided into 4 groups: 1, laboratory evidence of UTI on admission; 2, UTI just before admission; 3, UTI that developed during hospitalization; and 4, weak laboratory evidence of a UTI. The study included 90 TTP "visits." (A visit was defined as all admissions for TTP for a specific patient within a 1-month period.) Of the TTP visits, 21 (23%) were associated with UTIs. Group 1 included 7 patients; group 2, 10 patients; group 3, 3 patients; and group 4, 1 patient. This suggests that UTIs might serve as a TTP stimulus. Owing to the relatively strong association of UTIs with TTP, all patients with TTP should be screened for UTI and treated accordingly.