PURPOSE The need for surgical excision in patients with ultrasound-guided core needle biopsy (CNB)-diagnosed atypical ductal hyperplasia (ADH) remains an issue of debate. The present study sought to validate a scoring system (the U score, for underestimation) that we have previously developed for predicting malignancy in CNB-diagnosed ADH. METHODS The study prospectively enrolled 85 female patients with CNB-diagnosed ADH who underwent subsequent surgical excision. Underestimation was defined as a surgical specimen having malignant foci. RESULTS The overall underestimation rate was 37% (31/85). Multivariate analysis showed that a clinically palpable mass, microcalcification on imaging, size >15 mm and a patient age of ≥50 years were independently associated with underestimation. When applied to the scoring system, the validation score was significant (p<0.001; area under the curve, 0.852). No patient with a U score <3.5 had an underestimated lesion. CONCLUSION The present study successfully validated the efficacy of our scoring system for predicting malignancy in CNB-diagnosed ADH. A U score of ≤3.5 indicates that surgical excision may not be necessary.