Between 1980 and 1983, 689 women with primary breast cancer at the Royal Infirmary of Edinburgh and associated Hospitals had fine-needle aspiration biopsies prior to definitive surgery. Clinical factors relating to the success of these aspirations were evaluated. The most significant factor was which physician performed the aspiration. Size of the lesion was also an important variable; however, size difference could not account for the marked variation between different individuals performing the aspiration. There was no difference between node-positive and node-negative patients when matched by tumor size. A significantly lower rate of positive aspirations occurred in the 45 to 55 year age group which could not be accounted for by tumor size. Location of the mass was not significant, although there was a persistent lower rate of positive aspirates from the upper inner quadrants. Aspiration cytology was positive or suspicious in 65 percent of patients with primary breast cancer who had clinical diagnoses of benign breast lesions. It is concluded that the most significant variable in the accuracy of breast aspiration biopsy is the size of the lesion and the proficiency of the individual performing the procedure. With a skilled physician, positive aspiration results were obtained in over 80% of breast cancers.