BACKGROUND: Although low-dose aspirin is used by many elderly patients, monitoring of renal function is currently not recommended. We recently reported transient retention of uric acid and creatinine caused by aspirin in doses of 75 to 325 mg/d. We therefore evaluated the renal effects of aspirin (100 mg/d), including post-treatment effects. METHODS: We studied 83 stable geriatric patients in long-term care (aged 56 to 98 years) who were treated with low-dose aspirin (100 mg/d) for 2 weeks and 40 control patients. Other medications and diet were kept constant. Biochemical monitoring including blood samples and 24-hour urinary collections for creatinine and uric acid at baseline and weekly for a total of 5 weeks. RESULTS: After 2 weeks on aspirin, urinary excretion of creatinine decreased in 60 (72%) and excretion of uric acid decreased in 54 (65%) of the 83 patients, and their mean clearances decreased; during the same period, serum blood urea nitrogen, creatinine, and uric acid levels increased (P <0.05 for all). Deterioration from baseline levels was significantly greater (and more prevalent) in the aspirin-treated group than in the 40 control patients (P = 0.001 to 0.09). After withdrawal of aspirin these parameters improved. However, 3 weeks after stopping aspirin, 48% (35 of the 73 in whom this measurement was available) had a persistent decline in creatinine clearance from baseline, as compared with only 8% (3/36) controls (P <0.001). CONCLUSION: Short-term low-dose aspirin treatment may affect renal function in elderly patients. These effects persist 3 weeks after cessation of the drug in some of these patients.