Quantitation of planar radionuclide images is hampered by structures containing radioactivity which overlie or underlie the organ of interest. The introduction of single photon emission computerized tomography (SPECT) overcame this problem to a large extent and enhanced the contrast of the images. Attenuation of photons, however, degrades the resultant SPECT images and correction methods for photon absorption and scatter were subsequently proposed. The different correction methods have variable effects on the reconstructed images. If threshold techniques are used to quantitate organ volume, i.e., combining pixels with the same percentage of the maximum pixel count in the volume, the selected threshold values which give the most accurate volume determination, will be affected by the specific correction method used. In this study, the effect of various SPECT image correction methods on threshold was investigated. A thorax phantom containing volumes ranging from 30 to 1200 ml was used. Threshold values varying from 45.6% (210 ml without any correction) to 23.7% (1200 ml with a combination of scatter subtraction and attenuation correction) were used to produce correct quantitation when different methods were investigated. A negative correlation was found between threshold and volume. This reduction in threshold was most prominent when scatter and attenuation correction were combined. This study shows that correction methods for attenuation of photons influence the threshold value for volume quantitation and the use of a constant threshold value could lead to underestimation of larger volumes.