Mammary intraepithelial lesions (IEL) are nowadays frequently diagnosed as a result of the success of mammographic screening, education programs, and awareness by women. Establishment of an animal model for these lesions to test treatment or preventive modalities is a prerequisite for human clinical trials. A model for spontaneous IELs, especially for estrogen receptor (ER)-negative lesions, does not exist. This study describes the histologic and immunohistochemical similarity between human and canine mammary IELs. Mammary tumors from 200 dogs were classified and histologic sections of the excisional specimens were evaluated for IELs. IELs, found in specimens from 60 dogs, were categorized as adenosis, sclerosing adenosis, intraductal papilloma, sclerosing papilloma, ductal hyperplasia, atypical ductal hyperplasia (ADH), and ductal carcinoma in situ (DCIS; high, intermediate, and low grade). Most proliferative IELs without atypia were associated with benign tumors, whereas IELs with atypia (ADH and DCIS) were generally associated with mammary cancer. ER-alpha expression was significantly low or absent in most ADH and DCIS lesions as well as in their associated tumors. Ki67 expression was significantly higher in high-grade DCIS than in hyperplasia or low-grade DCIS. Two thirds of high-grade DCIS lesions were positive for HER-2. Canine mammary IELs were strikingly similar to those of the human breast. The frequency of IELs in the dog, their association with spontaneous mammary cancer, their pattern of ER-alpha and HER-2 expression, and their histologic resemblance to human IELs may make the dog an ideal model to study human ER-negative (both HER-2 positive and negative) breast cancer progression as well as prevention and treatment.