Intravenous ciprofloxacin (200 mg every 12 hours) was compared with intravenous ceftazidime (1 g every eight hours) as therapy for 62 episodes of severe infections occurring in 60 adult patients, all of whom failed previous antimicrobial therapy. The study was designed as a prospective, controlled, randomized, non-blinded trial in a tertiary university care center. A variety of infections including skin and skin structure, urinary tract, bacteremia, pneumonia, and intra-abdominal infections were treated. Clinical cure was achieved in 83.3 percent (25 of 30) of patients treated with ciprofloxacin and in 87 percent (26 of 30) of patients treated with ceftazidime (p = 0.4). Bacteriologic and overall responses were also similar in both treatment groups (p = 0.4 and 0.375, respectively). Intravenous ciprofloxacin administered twice daily is an effective treatment for severe infections caused by susceptible organisms.