Operative cholangiography is a commonly employed adjunct to biliary operation and is useful in identifying unsuspected common duct calculi. Spasm of the sphincter of Oddi from administration of anesthetic agents like fentanyl citrate (Sublimaze) and morphine sulfate and from operative manipulation may lead to misinterpretation of the cholangiograms and unnecessary surgical procedures that increase morbidity and mortality. Six case reports illustrate these hazards, and the influence of medications that exacerbate spasm of the sphincter of Oddi is emphasized. Recognition of the factors that cause spasm of the sphincter of Oddi will prevent unnecessary common duct explorations and reduce morbidity and mortality in biliary tract operations.