A three-dimensional brain phantom has been developed to simulate the activity distributions found in the human brain in the cerebral blood flow and metabolism studies employed in PET (positron emission tomography). The phantom has a single contiguous chamber and utilizes thin layers of lucite to provide apparent relative concentrations of 5, 1, and 0 for grey matter, white matter, and ventricles, respectively, in the brain. The phantom and an ideal image set were created from the same set of data. Thus, the user has a basis for comparing measured images with an ideal image set, which enables the user to make quantitative evaluation of the errors in PET studies with a data set similar to that obtained in patient studies.<>