PURPOSE: To evaluate using spiral CT the effect of spleen size on blood flow in the portal venous system andto know the usefulness of this evaluation. MATERIALS AND METHODS: Fifty-one patients without evidence on spiralCT scan of abnormality thought to affect portal venous flow presented between December 1994 and June 1995. We measured spleen size and Hounsfield units of portal vein in dual-phase, and calculated the ratio of the unit inthe portal phase to that in the arterial phase. Spleen size was measured, using the length of X-axis by that of Z-axis on spiral CT scan. We then measured the correlation between the two values. CT was performed with a Somatom Plus-S scanner(Siemens, Erlangen, Germany). A total dose of 120ml of non-ionic contrast material(Ultravist) was administered at a rate of 3ml/sec. Arterial and portal phase were obtained after 30 seconds and 60 seconds fromthe beginning of the contrast agent injection. RESULTS: The correlation between spleen size and contrast enhancement of the portal vein was relatively significant(Pearson's correlation coefficient(r)=0.41801). CONCLUSIONS: Spleen size significantly affects portal venous flow on spiral CT scan. The evaluation of spleensize and contrast enhancement of the portal vein could be useful in the differential diagnosis of diseases which affect portal venous flow.