A constant relationship was found between the mean CT numbers of the liver and spleen in 100 normal adults. This relationship was characterized by a mean CT number consistently higher for the liver (24.9 +/- 4.6) than for the spleen (21.1 +/- 4.1). The range for liver CT numbers was 16.7-37.2, and for the spleen it was 14.9-34.3. The mean liver-spleen CT number difference for all subjects was 3.8 +/- 2.1 (p < 0.001); in every instance, the livers exhibiting the high mean CT numbers were in subjects with high mean spleen CT numbers, with the same concordance for low mean CT numbers. This relationship between liver and spleen may be useful in the clinical setting in which a normal liver with low CT numbers must be differentiated from one in which the CT number is low because of fatty infiltration; the fatty liver will exhibit a lower mean CT number than the spleen.