The clinical usefulness of [15O] H2O was evaluated in 59 patients (16 patients with brain disease, 19 with neck disease, 4 with chest disease, and 20 with abdominal disease) . [15O] H2O was produced in a medical cyclotron (AMWR01, NKK Co., Tokyo, Japan) . The images by positron emission computed tomography with [15O] H2O were judged clinically useful (“fairly useful” or better) in 14 (88%) of the 16 patients with brain disease, all 19 patients with neck disease, all 4 patients with chest disease, and 19 (95%) of the 20 patients with abdominal disease (19 with chronic liver disease) . In 14 of the patients with liver disease, hepatic arterial blood flow and portal blood flow were measured in the same session. The mean portal blood flow in cirrhotic patients with varices was significantly lower than that in cirrhotic patients without varices. There were no side effects during or after the computed tomography. Positron emission computed tomography with [15O] H2O was clinically useful.