The occurrence of bacteremia in association with diagnostic or therapeutic ERCP was studied in 180 patients undergoing a total of 194 examinations. Nineteen (15%) of 126 diagnostic procedures and 18 (27%) of 68 therapeutic procedures were associated with bacteremia (p less than 0.1). Nine patients had polymicrobial bacteremia and a total of 16 species were detected. Different streptococci, mainly alpha-hemolytic, were the most common bacteria which were identified in 38% of the bacteremic patients. There were no significant differences with regard to the occurrence of fever, pancreatitis, or septic complications between the diagnostic and therapeutic groups of patients. Neither did the complication rate in patients with bacteremia differ from that in patients without bacteremia, whether the procedure was diagnostic or therapeutic. Complication rates did not differ between patients with and patients without pancreaticobiliary obstruction. However, the majority of patients with biliary stasis had drainage with relief of the obstruction at the time of the diagnostic ERCP. We conclude that general routine antibiotic prophylaxis is not indicated in patients undergoing diagnostic or therapeutic ERCP. The question whether such prophylaxis should be given with certain diagnoses or treatments, or in patients with valvular heart disease, remains to be answered in controlled randomized studies.