To clarify the significance of magnetic renonance cholangiopancreatography (MRCP) in the acute phase of acute cholecystitis, MRCP was carried out in forty-five patients with acute cholecystitis in their acute phase. The MR pericholecystic high signal was observed in 38 of the 45 patients (84%). Enlargement of the gallbladder, presence of gallstones, and impacted stones was seen in 71%, 53%, and 18%, respectively. The MR pericholecystic high signal was classified into four categories:type 0, not observed;type 1, a liner high signal;type 2, a band-like high signal;type 3, a radiating high signal. In patients who showed a type 3 MR pericholecystic high signal, 91% required percutaneous transhepatic gallbladder drainage, and most of the gallbladders were diagnosed as necrotic cholecystitis by histology. The accuracy of MRCP for the diagnosis of choledocholithiasis was 96%. It was suggested that MRCP for patients with acute cholecystitis in the acute phase provides useful information for planning the treatment.