Introduction: Bacterial ascension together with biliary obstruction can lead to acute cholangitis. The purpose of our prospective study was to examine the spectrum of bacterial colonisation of the bile during ERCP in patients with cholestasis. Methods: Since 8/99 50 patients with cholestasis(by ultrasound and bloodwork) underwent ERCP. After cannulisation of the common bile duct 10 ml of bile were aspirated and cultured. For positive results ( ≥ 104cfu/ml bile) MIC values were determined for Levofloxacin, Ceftriaxone, Piperacillin, Metronidazole. Within 15 minutes after the ERCP blood cultures were taken. Results: see table In all patients (27 m, 23 f, age 45-79 y) endoscopic drainage of the biliary obstruction could be achieved (41 benign, 9 malignant). 10 patients with positive bile cultures developed new or increasing fevers after ERCP and required antibiotic therapy. With negative bile cultures only 2 patients needed antibiotics. In vitro, all aerobic isolates were sensitive to Levofloxacin, all anaerobic flora to Metronidazole. Several isolates were resistant to Piperacillin and Ceftriaxone. Discussion: Cholangitis develops significantly more often in patients with positive bile cultures (≥ 104cfu/ml) even after complete drainage with ERCP, compared to patients with negative cultures. Antibiotic therapy with Levofloxacin/Metronidazole seems adequate coverage in these patients. Further study is needed to determine whether Levofloxacin reaches therapeutic levels in bile.