A scanning collimated line source for simultaneously acquiring emission and transmission data from a gamma camera has been developed. The line source is microprocessor-controlled and incorporates hardware to electronically window the spatial gamma camera signals in order to separate the emission signals of the subject from transmission signals from the line source. The device improves upon the previously described emission-transmission scanning technique using a flood source in three ways: (1) it overcomes the limitation that the transmission radionuclide must have a lower energy than the emission radionuclide; (2) it provides narrow-beam (scatter free) attenuation measurements of the subject being examined; and (3) it reduces the radiation exposure to staff. Attenuation coefficients for an elliptocal water-filled phantom were measured to be mu = 0.15 +/- 0.01 cm-1. The technique has been validated in phantom and human studies using a range of radionuclide combinations and imaging geometries and gives equivalent results using separate and simultaneous acquisitions.