A technique is described for the quantification of the three-dimensional distribution of 131I radiopharmaceuticals using single photon emission computed tomography (SPECT). A theoretical discussion of the problems of scatter and septal penetration in 131I SPECT is presented. The correction of these effects using the geometric mean line source scatter subtraction technique and a distance varying counting sensitivity, is described. The volume and activity of objects in the SPECT dataset were assessed by segmentation using the technique of maximization of interclass variance. The technique has been examined in phantom experiments showing that volumes and activities of objects containing a uniform concentration of activity can be assessed to a precision of 7.3% (1 S.D.) and 0.32 MBq (1 S.D.) respectively. The technique has been successfully applied to patient studies using both 131I-metaiodobenzyl guanidine (MIBG) and lipiodol.