A contour-finding algorithm is described by which the object periphery is outlined in positron emission tomography (PET) scans. The positions of maximum slopes are determined in the projections. These positions are shown to correspond well to the borders of the activity region. By backprojecting these points in the projections into the image plane, the size and shape of the region of activity must be outlined. Cord lengths across this region are then determined for attenuation correction. The accuracy of the algorithm has been tested on 10 patients with both PET and computed tomography (CT) examinations. Points on the periphery of the skull, as determined with the contour-finding algorithm in PET, coincided with those determined by CT, with a standard deviation of 1.5 mm and a maximum deviation of 3.5 mm. In testing the influence of the skull bone on the PET values, it was found that ignoring the higher attenuation coefficient of bone, 5 mm thick, in the attenuation correction caused an error in the determination of activity of 11% peripherally and 6% centrally in the object. These errors could be reduced to less than 1% by increasing the assumed attenuating region by 4.5 mm.