The application of fine needle biopsy as a tool for early detection of breast cancer is becoming extensive, therefore parameters reported to be associated with prognosis should be standardized in this material. We propose the sequential determination of estrogen receptor (ER) status and DNA ploidy on the same smear obtained from a fine needle biopsy of a breast carcinoma, since both parameters seem to reflect properties associated with tumor behaviour and biological aggressiveness. Fifty fine-needle biopsies were investigated for presence of ER by the monoclonal antibody D75 followed by DNA content quantification using Feulgen-DNA cytophotometry. Overall, 66% of the tumors showed immunoreactivity for ER and 66% were classified as aneuploid. Forty-one percent of the aneuploid tumors were negative for ER, while only 7% of the diploid tumors showed no immunoreaction (p less than 0.05). The significant association between absence of immunocytochemical ER and DNA aneuploidy on the same fine-needle smear is consistent with data obtained through other methods previously reported using much larger tissue samples.