The effect of neuroanatomical structure size, shape, and position versus spatial tomographic resolution on quantitation in positron computed tomography was investigated. For neuroanatomical structures, voxel sizes in excess of 3 ml exceeded the volume of most structures examined. When the voxel size exceeded structure volume, calculated recovery coefficient (fraction of the true isotope concentration measured in the image) fell to less than or equal to 42%. Partial volume effects in the plane of section analyzed by computer simulation produced errors that were largest for small, thin, irregularly shaped structures whose averaged pixel values were most different from neighboring structures. Smallest errors occurred in large, circular structures surrounded by regions of similar pixel values. Computer simulation of regional cerebral asymmetries of pixel values demonstrated that the measurement of these asymmetries was often predominated (enhanced or obliterated) by partial volume effects related to structure size and shape. Large, circular, and widely separated regional asymmetries were more easily detected at a given spatial resolution than small, thin, adjacent regions. Recommendations for error reduction and possible correction factors are provided and discussed.