The accessory pancreatic duct (APD) is sometimes developmentally obliterated near the duodenum. We evaluated patency of the APD by dye-injection endoscopic retrograde pancreatography (ERP). We injected 2–3 mL contrast medium containing indigocarmine into the main pancreatic duct (MPD) via a selectively cannulated endoscopic catheter. Patency of the APD was evaluated by observing the excretion of dye from the minor duodenal papilla. Of the 291 control cases studied, 43% demonstrated a patent APD. Patency of the APD in patients with acute pancreatitis was only 17%, significantly lower than that of controls (P < 0.01). Mean caliber of patent APD was 1.6 ± 0.5 mm, significantly greater than the 1.1 ± 0.5 mm of non-patent APD (P < 0.01). Regarding the terminal shape of the APD, spindle- and cudgel-type APD were frequently patent (93% and 88%, respectively, (P < 0.01). With respect to APD course, long-type APD showed most frequent patency (75%, P < 0.01). Dye-injection ERP represents a simple and definitive method for examining APD function. A patent APD may prevent acute pancreatitis by reducing pressure in the MPD. Patency of the APD might be dependent on duct caliber, course, and terminal shape.