Methods for determining spleen size in vivo have many clinical applications. Spleen size can be estimated accurately in vivo from measurements of length (L) and width (W) of the spleen silhouette as visualized by a scintillation camera after administration of technetium-99m (99mTc) sulfur colloid to the patient. Assuming mass and volume are proportional, spleen weight will depend on (L x W)3/2. Estimates of spleen size using these parameters obtained from scintigraphs were compared to actual determinations following splenectomy. Within the range of observations, the method had a standard deviation of 45 grams. It is safe, fast, and inexpensive.