Biopsies of mammographically detected nonpalpable lesions have resulted in increased numbers of diagnosed early breast malignancies. From June 1992 to September 1996 a total of 433 consecutive patients underwent 438 biopsies. The mean age was 55.7 years (range 30-82 years); 150 patients were younger than 50 years. Mammographic findings were classified as microcalcifications (C), masses (M), masses with microcalcifications (MC), architectural distortions (A), and stellate lesions (S). In 30 women two needles were placed to localize a lesion in the ipsilateral side and in 5 on the contralateral side. There were 182 (41.6%) biopsies performed for M, 144 (32.9%) for C, 78 (17.8%) for A, 25 (5.7%) for MC, and 9 (2.1%) for S. The overall malignancy rate was 34% (149/438). Thirty-four women (23%) who presented malignancy were younger than 50 years of age. From year to year, it increased from 27% during the first year to 51% during the fourth year. Altogether 100 (67%) patients had invasive carcinoma, 40 (27%) ductal carcinoma in situ (DCIS), 6 (4%) lobular carcinoma in situ, and 3 (2%) tubular carcinoma. Four patients had simultaneous bilateral palpable and nonpalpable carcinoma. Among the patients, 9 of 20 with previously operated breast carcinoma and 9 of 19 with other previous malignancies were found to have early breast carcinoma. The mammographic finding with high rates of malignancy were S 67%, MC 40%, M 34%, C 33%, and A 28%. A group of 11 of 110 (10%) patients had histologically proven axillary lymph node metastasis. Results from this large retrospective study of wire-guided localization biopsies showed a relatively high rate of malignancy (34%) and DCIS (27%).