OBJECTIVES: To compare the safety and efficacy of levofloxacin with that of ciprofloxacin for the treatment of chronic bacterial prostatitis. METHODS: In a multicenter, double-blind, active-control trial, 377 men with a history of chronic bacterial prostatitis, current clinical signs and symptoms, and laboratory evidence of prostatitis were randomized to treatment with levofloxacin 500 mg once daily or ciprofloxacin 500 mg twice daily for 28 days. The primary endpoint was microbiologic efficacy in the microbiologically assessable population. The Meares-Stamey "four-glass" procedure was used to obtain prostatic secretions and urine for culture. RESULTS: A total of 377 subjects received the study drug. The clinical success rates, including cured plus improved patients, were similar (75% for levofloxacin and 72.8% for ciprofloxacin; 95% confidence interval for the difference in the success rates: -13.27 to 8.87), as were the microbiologic eradication rates (75% for levofloxacin and 76.8% for ciprofloxacin; 95% confidence interval for the difference -8.98 to 12.58). Enterococcus faecalis and Escherichia coli were the most common isolates. The 6-month relapse rates were similar for both regimens. Both levofloxacin and ciprofloxacin were well tolerated, with similar rates of adverse events. CONCLUSIONS: Levofloxacin 500 mg once daily for 28 days is as effective as ciprofloxacin 500 mg twice daily for 28 days for the treatment of chronic bacterial prostatitis. Isolation of a high proportion of gram-positive organisms, as well as gram-negative pathogens, underscores the necessity of choosing an antimicrobial agent with broad-spectrum activity.