A prospective study was performed on a series of 42 patients with breast carcinoma. Immunocytochemical determination of receptors for estrogens (ER) and progesterone (PR) and flow cytometric DNA analysis (DNA-ploidy and the S-phase fraction) were performed on material obtained from fine-needle aspirates (FNA). The patients underwent subsequent surgery, and the same parameters were determined on the surgical samples. The correlation between the biochemical and the immunocytochemical methods gave for ER, r = 0.853, p < 10 (-6) and for PR, r = 0.830, p < 1.6 (-6). In addition, comparisons of FNA's with the surgical samples showed significant relationships for the DNA index (r = 0.96, p < 0.000001) and for the S-phase fraction (r = 0.88, p < 0.000001).