The role of aspiration cytologic examination in the diagnosis of carcinoma of the breast. A study of 2,000 patients who had undergone fine needle aspiration of a solid lump of the breast was conducted to determine the accuracy of this technique. During the period 1982 to 1989, all patients with a solid mammary mass, presenting to a specialized breast unit were initially investigated by fine needle aspiration cytologic examination (FNAC). To date, we present data on 2,000 consecutive aspirations with matching histologic diagnosis on the excised pathologic specimens. In particular, the relationship between tumor histologic factors and the accuracy of cytologic diagnosis was examined. The sensitivity of FNAC in the diagnosis of a malignant growth was 84 per cent; this represents first aspiration results only. The specificity was more than 99 per cent. The rate of acellular or unsatisfactory aspirations was 12 per cent. We had a positive predictive rate for diagnosis in carcinoma of the breast of almost 95 per cent. The diagnosis of lobular and in situ patterns of disease was questioned, with three of three in situ lesions and ten of 23 lobular lesions missed by FNAC. We do not recommend that FNAC replace excision biopsy and frozen section in the diagnosis of carcinoma of the breast. 