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The hormone receptor (HR) status of breast cancer is an important prognostic factor and predictive parameter of the response to hormone therapy. Enzyme immunoassay (EIA) is currently the standard for determination of HR, but immunohistochemistry (IHC) represents a potentially useful alternative. We used IHC to determine HR status in a large prospective study and compared the results to those obtained by EIA. This study was designed to determine which technique should be used in daily practice in our institution which manages a large number of patients. Oestrogen (ER) and progesterone (PgR) receptor status was evaluated in a prospective series of 793 infiltrating breast cancers by IHC in paraffin-embedded tissue sections, using antibodies 6F11 and 1A6, with a rigorous quality control of the methodology. ER were found to be significantly expressed in 81% of cases after IHC analysis and in 78% of cases by EIA. For PgR, the respective rates of positivity were 65% and 69%. The tumour HR level detected by either technique was significantly correlated with the value of tumour size, histological grade and S-phase fraction. A significant link was observed between the percentage of labelled cells after IHC analysis and the amount of protein detected by EIA. Critical analysis of discordance found that, in the group of invasive lobular carcinomas, the rate of HR positivity was higher with IHC (84%) than with EIA (45%) and that, in the overall population, IHC was more specific than EIA, since cases with nonrelevant positivity related to intraductal normal or neoplastic cells expressing HR could be discarded. The cost of IHC analysis was found to be about one-third of that of EIA. IHC is more sensitive, specific and economical than EIA. It should constitute the new standard technique provided that good quality assurance procedures are respected. [ABSTRACT FROM AUTHOR]