9 elderly and 9 younger adult patients, with proven post-operative lower urinary tract infection were treated with 400 mg of norfloxacin twice daily for 5 days. Pharmacokinetics of norfloxacin were measured on days 1 and 5. Compared to the younger adult patients, the elderly showed a decreased creatinine clearance and, following the last dose on day 5, an increased maximum plasma concentration of norfloxacin, an increased area under the concentration-time curve and a decreased total body clearance of norfloxacin. These results confirm that in elderly, as in younger adult patients, the pharmacokinetics of norfloxacin can be described by a linear model and accumulation of the drug during repetitive multiple doses is predictable. The differences between the two groups cannot be considered as clinically significant so that no dose change would be required in elderly patients within the range of creatinine clearance studied.