Roux-en-Y gastric bypass, whereby the stomach, the duodenum and the jejunum are circumvented, is the most commonly performed procedure for the treatment of morbid obesity. In healthy volunteers, moxifloxacin has an oral bioavailability of 86.2 ± 1.11% (Stass & Kubitza, 1999). However, no data are available on the impact of gastric bypass on the oral bioavailability of the antibiotic moxifloxacin. To evaluate the effect of gastric bypass on the absolute oral bioavailability of moxifloxacin. This was a single-centre, open-label, randomized cross-over study in 12 healthy volunteers (8 female, age 25–57 yrs) who underwent gastric bypass surgery at least 6 months prior to inclusion in the trial, and had reached a stable body weight (81.4 ± 14.9 kg; BMI = 27.7 ± 4.3 kg/m²). Each subject received 2 single standard doses of 400mg moxifloxacin orally or intravenously (as a 1-hour infusion) administered on 2 occasions separated by a washout period of 1 week. Serial venous blood samples were drawn up to 72h after dosing, and the moxifloxacin plasma levels were measured by HPLC with fluorescence detection. After oral dosing, moxifloxacin plasma concentrations reached a maximum (Cmax) of 3.57 ± 1.25 mg/ml after 1.75h (0.75–4.00). After IV dosing, Cmax and tmax were 4.82 ± 1.86 mg/ml and 1.03h (0.75–2.50), respectively. The mean areas under the plasma concentration time curve extrapolated to infinity (AUC¥) were 48.20 ± 14.91 h*mg/ml after oral dosing and 54.20 ± 15.56 h*mg/ml after IV dosing, resulting in an absolute oral bioavailability for moxifloxacin of 88.31 ± 1.06%. This study suggests that gastric bypass has no effect on the bioavailability of oral moxifloxacin.