Background: The objective of this ultrasonography study was to determine the differential points among small hypoechoic nodules (longest diameter < 1.0 cm) in breast parenchyma using mastectomy specimens. Materials and methods: We performed ultrasonography of 25 mastectomy specimens to identify 33 hypoechoic structures in areas of normal parenchyma. Markings were made with hook-wire and/or ink. The ultrasonographic findings were then correlated with the pathologic findings. Results: Ducts (n = 9) were tubular-shaped, anechoic and located near other larger ducts in the subareolar area with oblique-axis. Fat lobules (n = 7) were disk-shaped, with identical internal echogenicity to subcutaneous fat tissue. Fibrocystic changes (n = 7) showed a hypoechoic appearance similar to the echogenicity of fat, but with a more irregular shape. Venules (n = 4) were identified as multiple tubular structures that were relatively more tortuous than ducts and with thinner walls. The intramammary lymph node (n = 1) was an ovoid, well-defined nodule with mass effect. Among the nodular lesions (n = 3), there were no differential points between invasive ductal carcinoma, nodular fibrocystic changes or fibroadenomatous hyperplasia, especially when the lesions were irregularly shaped. Cysts (n = 2) were easily diagnosed by ultrasonography. Conclusion: Various small hypoechoic structures (longest diameter < 1.0 cm) could be differentiated from each other by ultrasonography; however, histopathologic diagnosis is needed when the structures have an irregular shape.