Abstract: Collagenous spherulosis is a rare, benign breast lesion occuring in less than 1% of benign breast biopsies. All previously reported cases have been discovered as incidental microscopic findings in association with a range of benign to malignant processes. The authors report the first case of collagenous spherulosis presenting as a palpable mass. Immunohistochemistry and electron microscopy performed on this lesion demonstrated the presence of two cell types: epithelial cells and myoepithelial cells with associated basement membranelike material. Collagenous spherulosis may mimic adenoid cystic carcinoma since the epithelial proliferation and spherule formation in collagenous spherulosis closely resembles the changes in adenoid cystic carcinoma. However, adenoid cystic carcinoma is an invasive lesion that is almost always palpable, while collagenous spherulosis is almost always an incidental microscopic finding. Our case illustrates that collagenous spherulosis can also result in a palpable mass, thus palpability of the lesion cannot be used to differentiate these conditions.