Résumé : We performed a randomized, double-blind, placebo-controlled trial of antibiotic prophylaxis before diagnostic and therapeutic endoscopic retrograde cholangiopancreatography (ERCP) in 82 uninfected patients with cholestasis due to biliary tree obstruction of single or multiple causes. The patients were assigned to receive piperacillin (4 g) or placebo three times daily; prophylaxis was started just before initial ERCP and was continued until biliary drainage was completely unobstructed (by one or more ERCP procedures); the maximal duration of prophylaxis was 7 days. Nine patients were withdrawn from the study, and five others could not be assessed because of protocol violations. The 68 clinically evaluable patients underwent 81 therapeutic ERCP procedures. Clinical success (defined as an absence of fever, cholangitis, and clinical signs of sepsis during the 48 hours after the last dose of piperacillin or placebo) was documented for 32 (94%) of the 34 patients given piperacillin and for 24 (71%) of the 34 patients given placebo (odds ratio, 6.66; P = .01). Complete biliary drainage influenced clinical outcome favorably (odds ratio, 5.0; P = .02). All seven instances of bacteriologic failure (bacteremia) involved patients in the placebo group (P < .01). We concluded that antimicrobial prophylaxis significantly reduces the incidence of septic complications after therapeutic ERCP among patients presenting with cholestasis.