PURPOSE: To analyse the ultrasonographic findings of normal thymus in infants and determine the pathologic findings. MATERIALS AND METHODS: Forty two infants without a history of chronic disease, tumor or steroid therapy were scanned in the axial planes with a 7.0 MHz sector transducer. Bilaterality, homogeneity, and the shape and intensity of thymic echogenicity were analysed; shapes were classified as one of three types, namely bilobate, quadrangular or triangular. The intensities of thymic echogenicity were compared with those of the liver and spleen under the same conditions (time-gain compensation, depth and gain). RESULTS: Bilaterality was seen in 36 cases, homogeneity in 35, homogeneity and bilaterality in 29, inhomogeneity and bilaterality in seven, and homogeneity and unilaterality in six. No case showed inhomogeneity and unilaterality. The shape was bilobate in 23 case, quadrangular in 13 and triangular in six, while the outer border was convex in 29 cases and straight in 13. In no case was echogenicity of the thymus greater than that of the liver or spleen. Transverse diameters were 21-47mm (mean, 37.0mm; SD, 9.4mm) and depths were 10-37.5mm (mean, 25.8mm; SD, 5.9mm). CONCLUSION: The usual findings of normal thymus in infants are bilaterality and homogeneity, convex or shaight outer border, and echointensity less than or equal to that of the liver and spleen. The possibility of thymic pathology is suggested when ultrasonography shows unilaterality, inhomogeneous echogenicity, and echogenicity greater than that of the liver and spleen.