Guidewire-directed biopsy is the predominant method of evaluating nonpalpable breast lesions detected by mammography. It is an accurate and safe procedure for obtaining a histologic evaluation of abnormal tissue, leading to early diagnosis and improved survival rates. This report reviews 526 needle localizations and biopsies of nonpalpable breast lesions performed in two community hospitals over a 39-month period and compares early results with those achieved in the last 13 months of the study. This review indicates that as experience in using this modality increased, the cancer detection rate increased and the percentage of invasive cancers decreased. The percentage of cancers detected in the final 13 months of the study (19%, Group II) was higher than in the first 26 months of the study (13%, Group I). Although microcalcifications were found in 24 per cent of the mammograms leading to a diagnosis of cancer, they did not prove to be a reliable indicator of malignancy in this study.