PURPOSE: To determine if a subset of atypical ductal hyperplasia (ADH) lesions diagnosed at 11-gauge, directional, vacuum-assisted, prone, stereotactic biopsy fit the "probably benign" definition 2% chance of being carcinoma at subsequent surgical excision. MATERIALS AND METHODS: Clinical, mammographic, and stereotactic biopsy features in 104 consecutive nonpalpable ADH lesions were correlated with the presence of carcinoma at lumpectomy. The results were analyzed with chi(2) statistic, with P < .05 indicative of significant difference. RESULTS: Surgical excision revealed carcinoma in 22 (21%) of 104 ADH lesions. The lowest incidences of carcinoma (each P < .02) were 16% (15 of 92) in patients with no personal history of breast carcinoma, 13% (nine of 67) when maximum lesion diameter was less than 10 mm, and 8% (three of 36) when 100% of the mammographic lesion was removed at stereotactic biopsy. CONCLUSION: No clinical, mammographic, or biopsy features alone or in combination could be used to define a substantial subset of probably benign lesions with a less than 2% chance of carcinoma at lumpectomy. (C) RSNA, 2002.