Needle-localized mammographic lesions. Results and evolving treatment strategy. From January 1981 to December 1987, 932 needle-localization breast biopsies were performed at our institution for mammographically detected abnormalities. We reviewed 531 needle-localization breast biopsy procedures performed during two periods (January 1981 to June 1984, n = 311; and January to August 1987, n = 220) to compare results and treatment patterns, and to determine the prevalence of the missed lesions. Mammographic abnormalities detected on routine screening accounted for a larger proportion of needle-localization breast biopsies in the later series (94 [30%] of 311 vs 94 [43%] of 220). However, the rate at which carcinoma was identified remained constant at 29% as did the percentage of cancers that were invasive (46% vs 51%). Overall, the rate of malignant diagnoses after needle-localization breast biopsy was lowest in asymptomatic women undergoing routine screening mammography (44 [24%] of 188) and significantly higher in women undergoing mammographic follow-up of the contralateral breast after treatment for breast cancer (28 [43%] of 65). There were seven missed lesions in 531 needle-localization breast biopsies, necessitating a second procedure in six and interval mammograms in one. 