The biopsy of 148 clinically occult lesions of the breast found at mammographic examination yielded 21 instances of carcinoma (14.2 per cent). Five malignant lesions were lobular or in situ carcinoma, 16 were infiltrating ductal carcinomas and 11 were minimal carcinoma of the breast. Carcinomas found by mammographic examination were smaller and less likely to be associated with metastases to the axillary nodes than carcinomas of the breast diagnosed by palpation during the same time period. Biopsy of the breast was performed by several methods: 1, quandrantectomy with the patient under general anesthesia (the results of 29 biopsies yielded five instances of carcinoma, 17.2 per cent); 2, generous excision of breast tissue with the patient under general anesthesia and the lesion being localized by geometric co-ordinates based upon the mammograms (the results of 35 excisions yielded four instances of carcinoma, 11.4 per cent), and 3, mammogram-directed needle localization (the results of 84 procedures yielded 12 instances of carcinoma, 14.3 per cent). Because needle-directed biopsy causes less distortion of the breast, we prefer it to other biopsy techniques in the evaluation of occult lesions of the breast.