The tomographic distribution of thallium 201 in the myocardium was studied with a gamma camera with 2 detectors (29 cms in diameter) with 37 photo multipliers each. The camera was connected to a PDP computer with a 256 Kbytes memory. Thallium 201 in a dose of about 74 MBq 2 mCi was used in normal subjects, patients with coronary artery disease and with cardiac hypertrophy. Tomographic sections were obtained every 1.2 cms in the sagittal and frontal projections. The distribution of the Thallium was uniform in normal and hypertrophic hearts. Focal defects were seen in C.A.D. Single-photon emission computerized tomography was instrumental in determining the extension of the damage of the myocardium to areas which appeared normal in the regular thallium 201 scintigrams.