Spleen size in patients with infectious mononucleosis (IM) was measured using ultrasound, and the findings were compared with data obtained in cases of acute viral hepatitis (AVH). The size was expressed as the spleen index (SI) or the product of the longitudinal and transverse diameters, expressed in cm2, of the maximum crosssectional area of the spleen. A normal value obtained from 28 healthy individuals was 15 & 7 cm2. The SI (mean & SD) of patients with AVH and IM were 38 & 7 cm2 and 88 2 26 cm2, respectively. When the SIs were divided into five groups-grade 0 (0-25), grade I (26-50), grade I1 (51-751, grade I11 (76-loo), and grade IV (loo+), 83.3% of the patients with AVH were graded as Grade 0 or I, whereas 88.9% patients with IM belonged to higher grades. Splenomegaly with a SI value over 75, grade I11 or IV, strongly suggests IM when IM and AVH are being considered in the absence of other potential causes of splenomegaly. Because the clinical features of IM sometimes resemble those of AVH, it is often difficult to differentiate the two entities. The measurement of spleen size could be a significant supplemental aid in the diagnosis of IM in primary medicine, before clinical data can be obtained. Indexing Words: Infectious mononucleosis * Spleen * Spleen size * Spleen index * Acute hepatitis