Collagenous spherulosis is a rare benign lesion that can microscopically mimic atypical ductal hyperplasia, cribriform ductal carcinoma in situ, or adenoid cystic carcinoma. Seventy-nine cases of collagenous spherulosis in women aged 36 to 90 years (mean age, 52 ± 11 years) were identified from the University of Texas M. D. Anderson Cancer Center pathology files from the last 10 years. The pathology reports and slides from 59 patients were reviewed. This study does not attempt to evaluate the overall incidence of collagenous spherulosis because only cases with an actual diagnosis of collagenous spherulosis in pathology report were reviewed. The aim of this study was to investigate the underlying pathology within collagenous spherulosis and its relation to other pathologic findings within the specimen. Collagenous spherulosis was present as multiple foci ranging in size from 0.1 to 0.3 cm in 44 (75%) of the 59 cases. Collagenous spherulosis was associated with benign lesions of the breast in 41 cases, with atypical ductal hyperplasia in 3 cases, and with lobular carcinoma in situ in 15 cases. Microcalcifications were found in lesions involved by collagenous spherulosis in 15 cases. In summary, although collagenous spherulosis was mostly associated with benign breast lesions, in our study it was also frequently seen in association with lobular carcinoma in situ. Collagenous spherulosis could present as a mammographically suspicious mass or density and could be associated with microcalcifications. Increased numbers of cases of collagenous spherulosis may be expected in the future as a result of breast cancer-screening programs. Caution is needed not to overinterpret collagenous spherulosis as atypical or ductal carcinoma in situ, especially on core needle biopsy specimens.