A diagnosis of atypical ductal hyperplasia (ADH) after breast core biopsy usually is followed by an excisional biopsy to exclude the presence of a more significant lesion. To determine whether the immunohistochemical expression of cyclin D1 (CyD1) and Ki-67 can aid in case stratification for the likelihood of finding ductal carcinoma in situ (DCIS) on subsequent excision, we immunohistochemically stained 21 consecutive ADH cases diagnosed by core biopsy, and proliferation indices (PIs) were calculated for each case. Fluorescence in situ hybridization to detect CCND1 amplification was performed in 10 cases. In 5 cases, DCIS (with or without invasive carcinoma) was identified in the subsequent excision. The mean PICyD1 and PIKi-67 for these cases were significantly higher than in the remainder (P = .03 and P = .05, respectively). The sensitivities of PICyD1 and PIKi-67 for the presence of DCIS on subsequent excision were 100%, and the specificities were 75% and 69%, respectively. The specificity of the 2 markers combined was 88%. The number of cells with CCND1 amplification was higher in cases with DCIS or ADH on subsequent excision. Immunostaining for CyD1 and Ki-67 might help stratify cases of ADH on core biopsy and identify patients unlikely to have DCIS found on excision.