To evaluate the role of fine-needle aspiration (FNA) cytology in the diagnosis of primary breast cancer, we reviewed our experience over a recent 5-year period at Harbor-UCLA Medical Center. A total of 590 aspirates with subsequent follow-up were documented. One hundred thirty-three primary cancers were histologically proved. Of the 133 cancers, 91 (68%) were diagnosed as malignant by aspiration cytologic examination; another 22 (17%) were reported as suspicious. Of the cancers, 8 (6%) had a benign cytologic diagnosis and 12 (9%) had unsatisfactory smears. No cytologic tests positive for malignancy were found to be benign on histologic examination. With an absolute sensitivity of 75% and a specificity of 100%, we conclude that FNA cytologic examination is highly accurate in the diagnosis of breast cancer. With no false-positive studies, the finding of a clearly malignant cytologic condition obviates the need for biopsy before mastectomy.