Five cases of prominent, diffuse dilatation of the main pancreatic duct caused by mucin-hypersecreting carcinoma are reported. Adenocarcinoma was noted mainly within one markedly dilated main pancreatic duct. On endoscopic retrograde pancreatograms, dilated main pancreatic ducts with amorphous or well-defined filling defects resulting from mucin (five patients) or tumor (one patient) were noted. The proximal portion of the main pancreatic duct alone was delineated in three cases because of reflux of contrast material as a result of the presence of excessive mucin and a patulous orifice of the papilla of Vater (major duodenal papilla). Diffuse dilatation of the main pancreatic duct was demonstrated or suggested on computed tomographic (CT) and ultrasound scans. Four patients who underwent operations are alive and free from carcinoma (mean, 50 months after diagnosis). Whenever prominent, diffuse dilatation of the main pancreatic duct is encountered on CT or ultrasound scans, endoscopic retrograde pancreatography is mandatory to permit confirmation or ruling out of mucin-hypersecreting carcinoma of the pancreas.