Several methods exist for determining oestrogen receptor status in breast carcinomas. Biochemical methods have been widely used for many years, but recently immunocytochemical methods have become available. We have compared the outcome of the biochemical and immunocytological analysis in 274 breast cancer patients. Fine needle aspirates from all the patients were investigated immunocytologically and 214 tumours were positive (78%) and 60 negative (22%). Biochemical data were available in 155 patients, and the concordance between the two methods was 88%. Most of the 119 carcinomas (43%) that were only investigated cytologically were too small to allow both histological and biochemical analysis. A minority of patients were not operated on because of high age, and/or impaired health, or because the tumour was inoperable. In our opinion, biochemical and immunocytological methods are equally sensitive and specific in detecting oestrogen receptor in breast tumour tissue. In fine needle aspirates there is the additional advantage of morphological assessment of malignant nuclei and the possibility of obtaining material from small lesions.