Context.—: Atypical ductal hyperplasia (ADH) is a challenging diagnosis defined by cytologic and architectural features that carries an increased risk of subsequent carcinoma when diagnosed in isolation. In addition, ADH may secondarily involve benign breast lesions, wherein it carries variable clinical significance.
Objectives.—: To review the diagnostic criteria and clinical significance of ADH in isolation and as it involves benign breast lesions, and to review the evolving literature on its molecular signature.
Data sources.—: Recently published studies that collectively examine ADH were reviewed.
Conclusions.—: Atypical ductal hyperplasia carries an increased risk of subsequent carcinoma in isolation and when it involves most benign breast lesions. Identifying which cases of ADH will be upgraded to carcinoma has been challenging, and new laboratory developments, such as EZH2 overexpression, may have a future role.