Purpose : To understand the cause of atypical sonograpic findings by analyzing their pathologic correlation to internal echogenicity of breast fibroadenoma. Materials & Methods : Between January 1995 and April 1997, the presence of 91 fibroadenomas in 81 patients was histopathologically proven. These mass lesions were sonographically 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 reviewed each 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 with pathologic findings. Results : There was significant correlation between increased vascularity and increased internal echo strength and between increased fibrosis and decreased internal echo strength. There was no significant correlation between internal echo homogeneity or posterior shadowing and vascularity or stromal fibrosis, 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.