UNLABELLED: The goal of this research was to develop and evaluate a practical transmission scanning system for attenuation correction on a 2-head gamma camera coincidence scanner. METHODS: The transmission system operates in singles mode and uses point sources of 137Cs that emit 662-keV gamma-radiation. Each point source is inserted between existing septa that are normally used to provide an approximately 2-dimensional emission acquisition geometry. The sources are placed along a line parallel to the axis of rotation near the edge of 1 camera. Data are acquired with the opposing camera. The septa provide axial collimation for the sources so that the transmission system operates in a 2-dimensional offset fanbeam geometry. Camera energy and spatial resolution were measured at 511 and 662 keV. Sensitivity was measured at 662 keV. The effects on axial resolution of adding supplemental collimation to the septa were shown. The system was calibrated and tested using a resolution (rod) phantom and a uniformity phantom. Torso phantom data were acquired. Patient transmission and emission scans were obtained. Postinjection transmission data were used to correct patient emission data. RESULTS: The camera resolution at postinjection counting rates was 11.7% full width at half maximum (FWHM) for 662-keV gamma-rays. Intrinsic spatial resolution was 2.7 mm (FWHM) at 662 keV. The sensitivity of the system was 280 Hz/MBq using five 74-MBq sources of 137Cs in the transmission geometry, with supplemental collimation added to the septa to improve axial resolution. The transaxial resolution of the system was such that the smallest rods (6-mm diameter and 12-mm spacing) were well resolved in a reconstructed resolution-phantom image. The corrected patient emission scans were free of attenuation-induced artifacts. CONCLUSION: An easily implemented transmission system for a 2-head gamma camera coincidence scanner that can be used for postinjection transmission scanning has been developed.