A 20-year-old woman who had been aware of a left breast mass since she was 19 years old presented to our hospital for the purpose of close exploration and treatment because it tended to grow. An elastic soft tumor 9.5 cm in diameter was palpated in the left BD area. Ultrasonography revealed a well-circumscribed solid tumor with a uniform inner part. The tumor partly presented a slit-like appearance, and it seemingly showed findings suggestive of fibroadenoma or a phyllodes tumor. However, a fine needle aspiration biopsy revealed only atrophic mammary tissue. Magnetic resonance imaging (MRI) scan revealed a low-intensity tumor on T2-weighted images and the enhancement effect was mild and ununiform, which were different from the typical features of fibroadenoma or phyllodes tumor. Without gaining the definite diagnosis, we performed tumor extirpation for the diagnosis and treatment. Histopathology showed the tumor to be an almost uniform lesion composed of proliferation of dense stromal tissue with a slit-like space and of poorly atypical mammary tissue. The stromal cells were positive for CD34 and SMA and negative for Factor VIII and p63 on immunostaining. Pseudoangiomatous stromal hyperplasia was thus diagnosed. Our experience with the disease is presented here with some bibliographical comments.