The association of abnormal nonmalignant cells in nipple aspirate fluid (NAF) and atypical proliferative disease (APD) of breast ducts was examined in the presence and absence of breast cancer in 134 patients. A high frequency of NAF samples with cellular abnormalities (atypical hyperplasia) was found when APD occurred with either breast cancer (70%) or benign breast biopsy specimens (54%). APD was present in 27 malignant cases (80%) and in 39 benign cases (39%). Inasmuch as APD may represent an intermediate step in the malignant transformation of duct epithelium, the cytologic examination of NAF offers a new approach with which to study the spectrum of breast cancer precursors and to assist in identifying women at risk for breast cancer.