The real-time ultrasonograms of 15 patients with pancreatic pseudocysts (10 infected and 5 noninfected) were analyzed to evaluate difference in ultrasound characteristics between the infected and noninfected pseudocysts. Only those who underwent ultrasound-guided aspiration or operation within one week after sonography were reviewed according to the size, multiplicity, air content, internal echoes, and wall characteristics (such as thickness, regularity, and calcification) of pseudocysts. Among these ultrasonographic features of pseudocysts, there was no statistically significant difference between the infected and noninfected pseudocysts in cyst size, wall characteristics (thickness, regularity, and calcification), multiplicity, and air content. The most important and unique feature was the internal echoes within the pseudocyst. The internal echoes were classified into three grades. All the infected pseudocysts and one noninfected pseudocyst had internal echoes of grade 2 or higher, while the most noninfected pseudocysts had internal echoes of grade 1. The difference is statistically significant ( p = 0.0037). These results indicate that grading internal echogenicity of the pseudocysts with real-time ultrasonographies can add additional information important in differentiating infected from noninfected pseudocysts. Indexing Words: Pancreatic pseudocyst * Abscess -Ultrasound ' p = 0.0037 (Fisher's exact test).