Nine cases of congenital choledochal cysts are reported which were correctly diagnosed by 131I-rose bengal or 131I-bromsulphalein (BSP) serial scintigraphy. They were classified by scintigraphic patterns into two types depending on whether biliary obstruction was complete or incomplete. All but two showed incomplete obstruction. The correct diagnosis may be difficult to make by scintigraphy if complete biliary obstruction is present. In such cases the clinical findings and other diagnostic studies, such as angiography or ultrasound, should be carefully considered. Serial scintigraphy with 131I-rose bengal or 131I-BSP is nontraumatic, safe, and provides increased diagnostic accuracy. It is the examination of choice when choledochal cysts are suspected clinically.