The clinical features of gallbladder carcinoma associated with acute cholecystitis and problems in diagnosing this condition by ultrasonography were investigated. Nine cases of gallbladder cancer with acute cholecystitis and 51 cases of gallbladder cancer without acute cholecystitis were reviewed. There were no obvious differences between those two groups as far as depth of invasion, location of cancer, and the prevalence of gallstones were concerned. However, there was a tendency for detection of cancer associated with acute cholecystitis by ultrasound to be more difficult than detection of cancer without acute cholecystitis. Macroscopic examination of resected specimens showed that superficial or flat type cancers tended to occur more often in cases of acute cholecystitis. This is considered to be the main cause of the difficulty in diagnosing such cancer by ultrasound.