Fine-wire localization biopsy is an accurate technique for diagnosis of mammographically-detected breast abnormalities, and may also be therapeutic in the management of impalpable malignant lesions. A number of factors were therefore examined regarding their influence upon therapeutic success in a consecutive series of 129 localization biopsies. Factors included type of radiological abnormality, method and accuracy of wire localization and pre-operative cytology. Primary malignancy was detected at initial localization in 64 cases (malignant:benign ratio, 1.11:1); 26 (41%) achieving adequate local tumour excision margins without further surgery, and 38 undergoing further surgery to clear margins (mastectomy in 23, further wide excision in 15). Therapeutic success was related to the accuracy of pre-operative needle localization (needle hook within 1 cm of target lesion in 26/26 (100%) therapeutic biopsies, compared to 29/38 (76%) non-therapeutic biopsies (P<0.01, Fisher's exact test)); and to pre-operative cytology (suspicious/malignant cytology in 15/24 therapeutic, compared with only 9/29 non-therapeutic biopsies (P=0.013, chi-squared)). Localization biopsy has a high diagnostic success rate and a therapeutic value dependent upon accurate pre-operative cytological diagnosis, supplemented by precise needle localization of the target lesion.