In a screening population of women, the mammographic characteristics for 68 cases of atypical ductal hyperplasia (ADH) diagnosed by needle core biopsy (NCB) were reviewed to seek mammographic findings which differentiate between ductal carcinoma in situ (DCIS) and ADH. A blinded analysis by two radiologists was performed for 48 cases with microcalcification. The mammographic findings were correlated with the surgical histological results of benign non-atypical, ADH and carcinoma (DCIS or invasive) to identify features which were associated with a higher or lower odds ratio (OR) for malignancy. Underestimates for malignancy occurred in 14 of 29 cases with granular calcification form (OR 7.9, 95% confidence interval (CI) 1.5-41) and 6 of 8 cases with segmental/linear branching distribution (OR 9.0, 95%CI 1.6-52). No malignancy was found at surgical excision in 16 cases with fine, rounded calcification. In conclusion, detailed assessment of calcification distribution and form gave helpful predictors for malignancy. Lesions with fine rounded calcification were always benign.