To analyze the effectiveness of fine needle aspiration (FNA) cytology in a multidisciplinary setting in rural Australia and to compare the imaging (mammographic and ultrasound) appearances and cytomorphologic findings with the final outcome. Prospective analysis of ultrasound-guided FNA cytology results from 426 women, aged 40-86 years, with screening-detected mammographic abnormalities. Cases of microcalcification, assessed mainly by stereotactatic core biopsy, were not included in the study. The FNAs were performed at a rural breast screening and assessment program in New South Wales, Australia, over a three-year period between May 1993 and May 1996. Imaging, FNA and combined imaging and FNA results from 426 women were as follows. The imaging diagnoses included 176 (41%) benign, 34 (8%) probably benign, 17 (4%) equivocal, 104 (24%) suspicious and 95 (23%) malignant cases. The FNA findings showed 59 (14%) no epithelial cells seen (nondiagnostic), 175 (41%) benign, 36 (8%) atypical, 41 (10%) suspicious and 115 (27%) malignant. Combined imaging and cytologic results comprised 224 (52.6%) benign, 10 (2.3%) atypical/equivocal, 59 (13.9%) suspicious and 133 (31.2%) malignant cases. All the malignant cases, by combined assessment, had malignant histology, and all the benign cases behaved in a benign fashion. In 80% of the suspicious lesions, the histologic diagnosis was malignant, but only 10% of the atypical/equivocal lesions had malignant histology. The positive predictive value of diagnosis of malignancy by combined imaging and FNA was 100%, and the false negative rate was 0%. Despite the recent surge in the popularity of core biopsy, FNA cytology of impalpable, mammographically detected lesions, when practiced in a multidisciplinary setting, is an extremely accurate test with high sensitivity, specificity, predictive values and efficacy. FNA cytology of the breast is a well-tolerated, relatively noninvasive test with a very low risk of complications. The sensitivity and positive predictive values for malignant and suspicious mammographic categories are also very high.