Background. The use of antibiotics as an adjunctive therapy in the treatment of periodontal diseases is of special interest to dental practitioners. In addition to using an appropriate antibacterial agent, clinicians may find it useful to determine the local and systemic concentrations of antibiotics in infected periodontal sites to reduce the levels of bacteria. The purpose of this study was to determine the serum and gingival crevicular fluid, or GCF, concentrations of systemic ciprofloxacin in patients with periodontitis. Methods. Ten subjects with chronic periodontitis received ciprofloxacin (500 milligrams) twice daily for five days. The authors collected GCF and serum samples immediately after administering the first dose (baseline = 0 hours) and at consecutive time points. The orifice method was used for GCF sampling, and 5 milliliters of venous blood was drawn for serum analysis. The authors used high-performance liquid chromatography to determine ciprofloxacin concentrations in GCF and serum. Results. The authors found that ciprofloxacin concentrations in GCF were significantly higher than concentrations in serum at two, four, seven, 24 and 120 hours. Ciprofloxacin reached the maximum concentration, or Cmax (3.72 micrograms/ mL), in GCF two hours after the initial dose was administered. The concentration decreased to 2.06 μg/mL 24 hours after the initial administration of the drug. Serum Cmax was 2-58 μg/mL at 1.5 hours, and the concentration decreased to 0.26 μg/mL at 24 hours. Conclusion. The results of this clinical study show that ciprofloxacin is found in GCF and its concentration in GCF is significantly higher than that in serum. Clinical Implications. Ciprofloxacin may be useful in treating patients with periodontitis because it reaches higher concentrations in GCF than in serum.