Patient movement between PET scanning sequences can produce misalignment between attenuation and emission scans. Such misalignment introduces errors in the emission image. This study evaluates the severity of these errors and their effect upon quantitation of regional myocardial activity. Myocardial FDG scans from 14 patients were reconstructed with simulated translational, rotational and out-of-plane patient movement. Eight myocardial regions from each patient were examined to determine the effect such misalignment might have on regional myocardial activity. A 2-cm shift between attenuation and emission scans produced up to a 30% change in regional activity. Some regions of the myocardium increased while others decreased for a given magnitude and direction of shift, producing anomalous regional myocardial inhomogeneities in the image. Such changes could easily cause qualitative and quantitative misinterpretations. We present data permitting the reader to assess the magnitude of this effect in his/her own clinical setting.