PURPOSE: To evaluate the results of mammography and ultrasound-guided localization biopsy of nonpalpable lesions, and the differences between them. MATERIALS AND METHODS: Between January 1999 and December 2000, localization biopsies of 441 nonpalpable breast lesions were performed immediately after preoperative localization using a wire hook. Localization was mammography guided in 241 cases and ultrasound guided in 200. The former group included clustered microcalcifications( 195/241, 80.9%), mass (22/241,9.1%) and mass with microcalcifications (24/241, 10.0%), while the latter were almost all mass, or mass with microcalcifications (198/200, 99%). Only two lesions (1%) showed clustered microcalcifications only, and these were previously demonstrated at mammography. RESULTS: Overall, 68 lesions (15.4%) were confirmed as malignancy. Forty-six of 241 mammography guided localization biopsies indicated malignancy: there were 28 noninvasive carcinomas (60.9%), 25 ductal cancers in situ (DCIS), one DCIS combined with lobular cancer in situ (LCIS), and two DCIS combined with microinvasion. Twenty-two of 200 ultrasound-guided localization biopsies revealed malignancy; five such lesions (22.7%) were noninvasive carcinomas. CONCLUSION: The malignancy rate and proportion of noninvasive breast cancers indicated by mammography and ultrasound-guided localization biopsy differed, and this was because the former involved mainly microcalcifications and the latter, masses.