To evaluate the efficacy of low-dose long-term treatment with ciprofloxacin in the prevention of recurrent urinary tract infections in patients with spinal cord lesions and neurogenic bladder dysfunction, a prospective, randomized, cross-over, placebo-controlled study was performed. The study period was 12 months long, including 6 months of treatment with 100 mg. ciprofloxacin at night and 6 months of placebo treatment. The study was completed by 18 men and 3 women, median age 38 years (range 19 to 73 years). Within the last 12 months before inclusion into the study, the patients had between 3 and 14 urinary tract infections (mean 5.8) treated with antimicrobial agents. The number of urinary tract infections treated with antimicrobial agents during 6 months of ciprofloxacin prophylaxis was 5 and during the 6 months of placebo treatment it was 59 (p < 0.00005) [corrected]. Fecal specimens showed supercolonization with ciprofloxacin resistant bacteria (Acinetobacter calcoaceticus) in 1 instance. No severe side effects were observed. Ciprofloxacin at a dose of 100 mg. at night was efficacious in preventing urinary tract infections during 6 months in patients with spinal cord lesions and neurogenic bladder dysfunction. After the controlled study 10 of the 21 patients used ciprofloxacin as prophylaxis for up to 39 months with a marked reduction in the pre-study infection frequency. In 1 patient ciprofloxacin resistant Escherichia coli was subsequently found in the feces.