The availability and cell content of aspirated breast fluid from 1,706 women were evaluated to determine the usefulness of breast fluid cytology as an indicator of breast disease. A newly developed aspirator was used to obtain cells adequate for diagnosis from approximately 50% of the women tested. Fluids were most readily available from women between the ages of 30 and 50 years. Although abnormal cytologies were observed in all age groups, the relative proportion of abnormal specimens as well as the degree of abnormality increased as a function of age. More women over 40 years of age with a high risk of breast cancer had cells classified in the more abnormal categories than did women in the normal risk group. To localize otherwise occult lesions, contrast ductography was performed on all women with very abnormal ductal cells. Women with atypical hyperplastic cytologies most commonly had benign and premalignant breast disease at subsequent biopsy. Of 27 women with fluids classified as suspected carcinomas 18 (66%) had small carcinomas. These observations strongly suggested that the presence of atypical cells in aspirated breast fluids has important clinical application for early detection of breast cancer. One limitation of cytology was that the technique rarely detected carcinomas greater than 1 cm.