Aspiration biopsy cytology of occult breast lesions by use of the "scouting needle". A prospective study of 261 cases. The increased use of mammography for the detection of impalpable breast lesions has resulted in a need for new diagnostic techniques. In a prospective study of 261 occult breast lesions, aspiration biopsy was done with a standard fine needle and syringe. This "scouting needle" technique permits the collection of diagnostic cytologic material in the clinician's office without specialized radiologic equipment. Thirty of 53 malignant lesions (57%) were detected cytologically. A benign cytologic diagnosis, which was made in 220 patients, did not preclude additional investigation. Aspiration biopsy using the "scouting needle" is a rapid, low-cost, easily performed, initial procedure which complements "watchful waiting." When diagnostic of carcinoma, it results in immediate surgical intervention and may obviate two-stage surgical treatment. 