Oral ciprofloxacin has been shown to be effective in the treatment of infections due to gram-positive cocci and gram-negative rods. The efficacy and safety of intravenous ciprofloxacin was compared with that of intravenous ceftazidime in the treatment of 59 patients with well-documented serious infections in a prospective, controlled, randomized study with a third-party blinding. Thirty-three patients were treated with intravenous ciprofloxacin (200 mg every 12 hours, plus a daily extra placebo dose); 26 patients were treated with ceftazidime (1 g every eight hours). The severity of the infections, underlying diseases, and demographic features were comparable in both groups, although there were more men in the ciprofloxacin group. For ciprofloxacin/ceftazidime treatments, respectively, the evaluated infections were: pyelonephritis (16 patients/nine patients), pneumonia (three/five), soft-tissue infections (four/zero), spontaneous peritonitis (five/two), primary bacteremia (three/eight), and other (two/two). Isolated pathogens included: Escherichia coli (22/12), Klebsiella sp. (five/four), Pseudomonas aeruginosa (two/three), Haemophilus influenzae (one/one), Proteus mirabilis (two/zero), Proteus vulgaris (one/zero), Salmonella sp. (zero/two), Plesiomonas shigelloides (one/zero), and others (one/four). The clinical responses were cure or improvement in 31 ciprofloxacin cases/21 ceftazidime cases; failure, zero/four; and indeterminate, two/one. The bacteriologic responses were eradication in 28 ciprofloxacin cases/22 ceftazidime cases; persistence, one/three; and indeterminate, four/one. Mild intolerance occurred in three ciprofloxacin cases and two ceftazidime cases. A mild increase in serum hepatic enzymes was observed in two patients in each group. Superinfections occurred in five patients: enterococcal septicemia (zero/two) and urinary tract infections (one/two). The results presented suggest that intravenous ciprofloxacin is an effective and safe antimicrobial agent for the treatment of serious infections, with an efficacy comparable with that of ceftazidime, a broad-spectrum cephalosporin. An additional advantage seems to be a lower rate of superinfections.