Nodular mucinosis is an extremely rare breast lesion. This benign mass usually presents clinically as a poorly circumscribed, subareolar, myxoid mass in young female patients. We report a case of this rare breast lesion and discuss its clinical, radiologic, and histopathologic features. A 21-year-old white woman presented with a mass in the left breast of 6 months' duration. She had never been pregnant or had any history of breast feeding, surgery, trauma, or use of exogenous hormones or a family history of breast cancer. Clinical breast examination demonstrated a 1 cm "rubbery" mass directly under and continuous with the left nipple. The skin that covered the mass had an edematous and irregular appearance without erythema or drainage from the nipple. Ultrasonography demonstrated a 1-cm, nonintraductal, circumscribed, homogeneous, isoechoic mass that was continuous or part of the base of the left nipple. The mass was smooth, with a thin echogenic rim. Doppler flow showed some vascularity. These findings suggested a benign breast lesion, including a fibroadenoma or nipple adenoma. Despite reassurance, the patient desired excision of the lesion. Gross examination revealed a nodular, rubbery-firm, ovoid, pink, polypoid mass that measured 1.5 x 0.9 x 0.8 cm. Microscopic examination showed a well-circumscribed tumor with a nodular appearance, which consisted of an accumulation of pink myxoid tissue and contained spindle cells with bland-appearing nuclei, no mitosis, and mild cellularity. The pink myxoid tissue was stained with Hale colloidal iron and Alcian blue. The Alcian blue stain was removed by pretreatment with hyaluronidase. The spindle cells stained with vimentin and smooth muscle actin; however, they did not express smooth muscle myosin or cytokeratin. This report presents and discusses the pathologic, ultrasonographic, and clinical findings of this rare entity.