1768 fine needle aspiration biopsies (FNAB) of the breast were evaluated between January 1981 and June 1984 at the Cytologic Laboratory, Institute of Pathology, Cantonal Hospital, St. Gall. The number of FNAB increased from 334 in 1981 to 837 in 1984. By far the most frequent indication for FNAB was a palpable lesion of the breast (in 92.1% of all cases). Other indications were radiological lesions without a palpable mass (2.7%), skin changes of the breast (1.7%) and, in a few cases, unilateral enlargement of the breast. Sensitivity and specificity of the method were calculated on the basis of two groups (A and B). In group A the FNAB was performed by a cytopathologist and in group B by general practitioners and specialists or hospital doctors. Sensitivity of 90.0% was calculated in group A and of 79.0% in group B. The specificity in the evaluated material was 100% for both groups. In group A, 5% of all carcinoma showed a false negative cytologic diagnosis, while in 9% of all FNAB the aspirated material was not representative. Most of the false negative diagnoses occurred with scirrhous carcinoma and with small carcinoma less than 1.5 cm in diameter. The percentage of the cases with a non-representative diagnosis reached 5% of all histologically proven carcinoma in group A and 19.2% in group B. The significantly higher percentage of non-representative diagnoses, together with a significantly lower sensitivity in group B (A: 90.0%, B: 79.0%) lead to the conclusion that the experience of the person who has performed the aspiration is of great importance for the diagnostic result.(ABSTRACT TRUNCATED AT 250 WORDS)