The greatest limitation of 201Tl for the diagnosis of coronary artery disease remains the high soft-tissue attenuation of its low energy photons. However, thallium also emits a smaller number of higher energy photons. Since photons of higher energy undergo less attenuation, it was postulated that the amount of attenuation could be ascertained from the ratio of the low-to-high energy photon counts. A planar phantom was used to derive a regression equation relating attenuation to the count ratio of the low and high energy peaks. A three-dimensional heart phantom was constructed and SPECT images were obtained in air and water. Application of the attenuation correction algorithm to the SPECT images of the phantom in water produced images quantitatively similar to that obtained in air. We conclude that by using differential photon attenuation, correction of 201Tl SPECT using a single order post-processing technique is feasible. This technique may allow for more accurate analysis of thallium myocardial perfusion scintigraphy.