Clinical and bacteriological data from 55 patients who developed septicemia within 3 days after ERCP were collected. Forty-four patients presented with septicemia after therapeutic endoscopy, with incomplete drainage in forty, eight after diagnostic ERCP performed in obstructed bile ducts in another center and not followed by endoscopic therapy, and three with a normal common bile duct after diagnostic ERCP. The incidence of septicemia is significantly higher in cases of malignant obstruction than in benign obstruction (21% vs 3%; p less than 0.01), due mainly to the problems of drainage associated with tumoral infiltration. Forty-eight patients (87%) had incomplete bile duct drainage when they developed septicemia, and among the seven remaining cases, 3 had cholecystitis and 3 abscesses in the biliopancreatic area. Previous diagnostic ERCP without drainage was also clearly associated with septicemia after therapeutic ERCP. The most commonly isolated bacteria from blood and bile cultures were Pseudomonas aeruginosa and Escherichia coli. P. aeruginosa was observed mainly in patients referred from other centers after previous diagnostic ERCP, and was unusual in patients without previous ERCP. It is associated with problems in the disinfection of the scopes. Six deaths were attributed to sepsis, always in patients with incomplete biliary drainage which could not be improved. In most of the cases, septicemia after ERCP is related to incomplete bile duct drainage, and in some cases, to biliopancreatic infected collections. Careful disinfection of the endoscopes and other endoscopic devices is mandatory to avoid an unacceptably high rate of P. aeruginosa infection.