PURPOSE: To understand the cause of atypical sonograpic findings by analyzing their pathologic correlation tointernal echogenicity of breast fibroadenoma. MATERIALS AND METHODS: Between January 1995 and April 1997, thepresence of 91 fibroadenomas in 81 patients was histopathologically proven. These mass lesions weresonographically interpreted and their descriptive criteria-internal echo content (both strength and homogeneity),the presence of septum, bilateral shadowing, and posterior echo pattern-were tabulated. A pathologist reviewedeach case and independently recorded the following data : cell type, the presence of septum, duct dilatation,calcification, fibrosis, hyalinization, and vascularity. We analyzed the correlation of sonographic withpathologic findings. RESULT: There was significant correlation between increased vascularity and increasedinternal echo strength and between increased fibrosis and decreased internal echo strength. There was nosignificant correlation between internal echo homogeniety or posterior shadowing and vascularity or stromalfibrosis, nor between hyalinization or cell type and internal echo strength, homogeneity or posterior shadowing.There was correlation between absent or thin capsule and the absence of bilateral shadowing. CONCLUSION: Increased vascularity or decreased stromal fibrosis might be the cause of atypical fibroadenoma.