(123)I imaging is increasingly used in nuclear medicine but downscatter from high (>300 keV) energy emissions degrades the image and introduces variation in sensitivity with depth when imaging with a low energy collimator. A dual windowing technique using a matched window immediately above the principle window to correct for the effects of downscatter is described. The technique is shown to correct for the variation in sensitivity with depth and to reduce outlying scatter. Quantitative imaging of (123)I using a low energy collimator should always employ downscatter correction.