It has been very difficult to differentiate inflammatory pancreatic mass from pancreatic cancer even by endoscopic retrograde pancreatography and angiography. We examined 10 patients with tumor-forming pancreatitis and 42 patients with pancreatic cancer by ultrasound, and differential diagnosis of these was attempted. The ultrasonogram of the inflammatory mass showed hypoechoic mass similar to the cancer, but the margin of the inflammatory mass was smooth shaped in 70% and unclearly demarcated in 90%. In addition, ductal structure in the tumor was seen in 9 patients with inflammatory mass in contrast to only 4 patients (14.2%) out of 34 with pancreatic cancer. Caudal pancreatic duct was demonstrated as irregular shape in caliber, even if not dilated, in 8 patients with inflammatory mass contrary to rosary-like dilatation in most of the patients with pancreatic cancer. The main pancreatic duct running into the mass (so-called duct penetration sign) was seen in 70% of pancreatitis in contrast with 11.1% of cancer. Therefore, the smooth shaped and unclearly demarcated margin of the tumor, ductal structure in the tumor, and duct penetration sign seemed to be specific to the inflammatory mass. However, it remains difficult to differentiate diffuse type or mucus-producing type carcinoma from chronic pancreatitis.