Ciprofloxacin is considered to be the standard treatment for patients with complicated urinary tract infections (UTI). This multicentre, randomized clinical study was designed to compare a once-daily regimen with 500 mg to the usual twice-daily regimen with 250 mg orally for 7-20 days. A total of 215 patients with significant bacteriuria (> or = 10(5) c.f.u./ml for females and > or = 10(4) c.f.u./ml for males) were included in the study. Eradication of bacteriuria was shown in 84.0% of patients treated with 500 mg q.i.d. and in 90.9% of patients treated with 250 mg b.i.d. Clinical cure or improvement was achieved in 97.3% of those treated with 500 mg q.i.d. versus 95.5% of those treated with 250 mg b.i.d. More superinfections occurred in patients treated with 500 mg q.i.d., mostly caused by gram-positive cocci. The statistical analysis concerning the elimination of bacteriuria using the 95% confidence interval showed a difference of up to 18.4% between the two treatment groups in favour of the 250-mg b.i.d. regimen. Ciprofloxacin was well tolerated with only mild to moderate side-effects (mostly gastrointestinal) in 8% of the patients. When using ciprofloxacin in patients with complicated UTI, its administration in two divided doses can be further recommended as a standard regimen.