Compared to other techniques, the ability to detect estrogen and progesterone receptors by immunocytochemical analysis in formalin-fixed, paraffin-embedded sections has clear advantages, including the ability to assay small biopsy specimens, fine-needle aspirate samples, and archival material. Twenty-two cases of breast carcinoma were evaluated for estrogen and progesterone receptors by immunocytochemical analysis and enzyme immunoassay. Using a true color-based image analysis system, histograms of area versus the optical density of the positive staining nuclei were generated. A binary decision algorithm was derived from these histogram parameters by the CART computer program. Estimates generated by the algorithm for image analysis/immunocytochemical analysis had a 90% concordance with the enzyme immunoassay values. It is concluded that quantitative immunocytochemical results for estrogen and progesterone receptor content in formalin-fixed, paraffin-embedded tissue can be generated using image analysis.