Based on limited reports of the successful use of antibiotics in the treatment of Crohn's disease (CD) and on the possibility that intestinal bacteria may be one of the etiologic factors playing a role in the pathogenesis of this condition, we undertook a study to evaluate the use of a broad-spectrum antibiotic in CD. Our team studied the efficacy of adding the antibiotic ciprofloxacin to the treatment of moderately active, but resistant cases of CD. Forty-seven adults with moderately active CD were randomly assigned treatment with ciprofloxacin 500 mg twice daily versus placebo twice daily for 6 months. The primary endpoint was the change in scores on the Crohn's Disease Activity Index (CDAI) from baseline to month 6. Although 47 patients were randomized, at 1 month of follow-up 28 patients received ciprofloxacin and 19 received placebo. The mean entry CDAI scores were not significantly different: 187 for the ciprofloxacin group versus 230 for the placebo group (p = 0.638). Mean CDAI scores at the completion of study were 112 for the ciprofloxacin group (n = 25) and 205 for the placebo group (n = 12), (p < 0.001). Disease remission is defined as a decrease in the CDAI score to less than 150 points. Our preliminary study suggests that ciprofloxacin may be an effective agent when added to the treatment of moderately active, resistant CD.