Purpose: Oral ciprofloxacin has the requisite pharmacokinetic and antibacterial properties to rival the potency of intravenous antibiotics. This study was designed to determine whether oral ciprofloxacin could abbreviate the course of intravenous antibiotics in the treatment of serious infections., Patients and Methods: Hospitalized adult patients were eligible for enrollment if they had a serious infection that was expected to require 8 or more days of intravenous antibiotic treatment. After conventional intravenous antibiotics were administered for 3 days, informed consent was obtained and patients were randomly assigned to either continue parenteral antibiotics (n = 53) or switch to oral ciprofloxacin 750 mg taken twice daily (n = 52). Ninety-nine of the 105 patients were evaluable for the assessment of efficacy. Clinical and bacteriologic efficacy, adverse events, and costs of the two treatments were compared., Results: The two treatment groups were comparable for demographic characteristics, types of infections, bacteria isolated, initial intravenous antibiotic regimens, and duration of antibiotic treatment. The most common infections were of the skin and skin structure; bacteremia and infections of the lower respiratory tract, urinary tract, and bone and joint were also represented. The most commonly isolated pathogens were Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. The most frequently prescribed intravenous antibiotics before randomization included aminoglycosides, cephalosporins, vancomycin, and nafcillin; 52 evaluable patients were treated with combination therapy while 47 received monotherapy. The clinical and bacteriologic outcomes and adverse reaction frequency with oral ciprofloxacin were comparable to those of the continued intravenous antibiotic regimens. Ciprofloxacin was associated with an average cost savings of $293 per patient., Conclusion: When used after 3 days of intravenous antibiotics, oral ciprofloxacin was as safe and effective as full courses of intravenous antibiotics and provided substantial cost savings.