It is the practice in some centres to use dual isotope imaging to reduce imaging times in lung ventilation and perfusion studies with 81Krm gas and 99Tcm-macroaggregated albumin (99Tcm-MAA) by simultaneous acquisition of the two images. The resulting loss of image caused by cross-talk between the two energy windows was investigated using two phantoms, one with cold 99Tcm lesions of varying size and contrast, and the other a uniform field of 81Krm. It was found that, under scatter conditions typical of a patient study, the use of dual isotope acquisition and a krypton generator of 470 MBq or greater resulted in a perceptible loss of image quality with lesions up to 4 cm in diameter being missed. On an older camera system, without modern energy and linearity correction facilities, a lower generator activity of only 120 MBq was sufficient to cause image degradation even under very low scatter conditions. Seventy-five patient studies were performed using both single and dual isotope imaging with generator activities ranging from 80 to 282 MBq. At these low generator activities, the studies did not demonstrate any differences between the images that would result in a different diagnosis. We conclude that the use of dual isotope V/Q scanning reduces the diagnostic value of the perfusion image if the activity of the 81Krm generator is too high, although at generator activities of 300 MBq or less no loss of image quality will occur on modern camera systems.