Eleven elderly patients (mean age 83 years; range 75-90) with microbiologically proven urinary tract infections were given 400 mg lomefloxacin as a single daily dose for up to seven days. On the first and final day of treatment blood was taken at timed intervals and drug concentration-time curves plotted. Blood was also taken immediately before each of the other doses for assay of pre-dose concentrations. The mean (+/- S.D.) peak serum concentration of lomefloxacin on the first day was 4.8 mg/l (+/- 1.5) observed at a mean of 156 min (+/- 88) and on the final day was 6.3 mg/l (+/- 2.5) at a mean of 119 min (+/- 68). The mean serum half-life on the first day was 10.0 h (+/- 2.8) and on the final day 10.3 h (+/- 2.5). The daily pre-dose serum concentrations of lomefloxacin showed no accumulation of the drug. No serious adverse events were reported and all patients were cured although two had persistent pyuria. It is suggested that a once daily dose of 400 mg lomefloxacin is suitable for the elderly and that no dosage modification is needed in this patient group.