Computed tomography potentially offers the most accurate noninvasive means of estimating in vivo volumes. Contiguous 1-cm-thick CT scans were obtained through phantoms, dog kidneys in vivo, and human spleens before splenectomy. Cross-sectional areas were calculated for each individual scan and volumes then determined with each of four mathematical integration techniques. Volume estimations were compared to volumes determined by water displacement. The simplest, most practical means of calculating volumes, using the summation-of-areas technique with scans obtained at 2 cm intervals, was similar in accuracy to more complex methods. The mean percentage error of volume calculations using the sum-of-areas technique was 4.95% for five immobile phantoms, 3.86% for eight dog kidneys, 3.59% for eight human spleens in vivo at 1 cm scan spacing, and 3.65% for the same human spleens at 2 cm scan spacings. Difficulties in visual recognition and manual tracking of object boundaries seem to be more significant sources of error than patient-related factors.