A premalignant lesion of the breast. Atypical ductal hyperplasia is often found in biopsy specimens of patients with breast cancer. It is characterized by a small number of breast ducts with abnormal cellular and glandular structures. Pathologists hypothesize that atypical ductal hyperplasia is one of several breast lesions that can develop into ductal carcinoma in situ and, ultimately, into an invasive cancer.