BACKGROUND Rest thallium-201/stress technetium 99m sestamibi protocol is widely used in the clinical setting. Although attenuation correction (AC) represents an important recent development in cardiac single photon emission computed tomography (SPECT) imaging, adjacent extracardiac activity can affect the myocardial count density distribution on AC images, particularly with 201Tl. The aims of this study were to compare normal distribution between AC rest 201Tl and stress 99mTc-sestamibi SPECT images as well as to evaluate the effect of extracardiac activity on AC SPECT images with 99mTc and 201Tl. RESULTS A phantom measurement and a study of 21 patients with low likelihood of coronary artery disease were performed with a triple-head SPECT system equipped with a americium 241 line source. In the phantom study, the presence of extracardiac activity increased the inferior-to-anterior ratios, particularly with 201Tl (1.01 to 1.32). In the clinical data, reduced count density with 201Tl compared to 99mTc-sestamibi was observed in most of the noninferior segments. On an individual segment basis, 37 (20%) of 189 segments from 11 (52%) of 21 subjects showed reduced count density on the 201Tl image compared to 99mTc-sestamibi by >10% of peak activity. CONCLUSIONS There is a significant difference in myocardial count density distribution between 99mTc-sestamibi and 201Tl on AC SPECT images, indicating that a careful image interpretation that considers the different normal count density distribution between the tracers and/or a tracer specific normal database is necessary, especially when defect reversibility is of concern. Further work should aim for the incorporation of scatter correction combined with attenuation correction.