Thirty-four patients were treated with intravenous ciprofloxacin. Thirty infections occurring in 28 patients were assessable for the efficacy analysis. The drug dosage was 300 mg every 12 hours in 19 patients and 200 mg intravenously every 12 hours in nine patients. Twelve patients were also given ciprofloxacin orally after initial intravenous therapy. The mean duration of total therapy was 31 days. The overall clinical response rate was 87 percent, and the bacteriologic response rate was 70 percent. Favorable responses were observed in 10 of 12 patients with osteomyelitis/septic arthritis; seven of eight with soft tissue infection; four of four with pneumonitis; one of two with cystic fibrosis; and four of four with urinary tract infections. Resistance to ciprofloxacin developed in three Pseudomonas aeruginosa isolates. Toxicity was minor: phlebitis occurred in six patients, nausea in six, and rash in one. Intravenously administered ciprofloxacin or intravenous ciprofloxacin followed by oral ciprofloxacin is a safe and effective therapy for serious infections.