Ultrasonography was used to evaluate spleen size in patients with various clinical conditions including those of the liver, blood, collagen, or autoimmune disease. To express spleen size, a spleen index (SI), the product of the transverse diameter and its perpendicular diameter measured on the maximum cross-sectional image of the spleen, was used. SI correlated well with the volumes of resected spleens. Splenomegaly was present in high percentages of patients with liver, blood, collagen, and autoimmune disease, even though a majority of these spleens were not large enough to palpate. By grading the SI, characteristic distributions of SI were obtained for patients with different types of diseases. Obtaining and grading the SI by the use of ultrasound appears to be a significant supplemental aid for evaluating spleen size, especially in patients whose spleens are not palpable.