Hemolytic uremic syndrome (HUS) is a rare condition that occasionally is reported in cancer patients. Recently it has been observed that gemcitabine rarely may be associated with this condition. The manufacturer's safety database and literature were reviewed for any report regarding gemcitabine associated with renal and hematologic abnormalities. Descriptive analysis was used to examine each case for an association between gemcitabine therapy and HUS and to identify its incidence and risk factors. Through December 31, 1997, 12 cases were identified that fit either the clinical (uremia, microangiopathic hemolytic anemia, and thrombocytopenia) or pathologic (renal biopsy) criteria for HUS. There were 7 males (58%) and 5 females (42%) with a median age of 55.5 years (range, 37-73 years). The median duration of gemcitabine therapy was 5.8 months (range, 3.8-13.1 months). Six patients died, five improved, and one patient's outcome was unknown. Among the six deaths, three patients died of cancer progression, one patient died of an unrelated myocardial infarction, and two patients died of HUS or HUS-related complications. For the five patients who improved, treatment was comprised of dialysis, plasmapheresis, splenectomy, or a combination. Attempts to correlate patient demographics, primary malignancy, and cumulative gemcitabine dose failed to identify consistent risk factors in predisposing patients to HUS. Confounding factors were common, including mitomycin-C and/or 5-fluorouracil exposure, advanced stage tumors, or preexisting renal dysfunction. Based on a patient exposure of 78,800, a crude overall incidence rate of 0.015% (range, 0.008-0.078%) was determined, showing that HUS associated with gemcitabine treatment appears to be rare. Nonetheless, as with other cancer treatments, clinicians should weigh the appropriate risk/benefit ratio in using gemcitabine to treat their patients.