Quality control investigations of estrogen and progesterone receptor (ER and PgR respectively) analyses can be undertaken using two principally different methods. The first, and the most frequently employed, is by comparing the results of analyses of a lyophilized, standard cytosol. The second method is by comparing the frequency of receptor positivity as well as the distribution of the receptor concentrations determined on a large number of representative biopsies in different laboratories. This latter method can, however, only be used provided that the composition of the population of biopsies studied is similar in the laboratories undertaking such an investigation. Both approaches have been used by the three Danish laboratories participating in the Danish Breast Cancer Cooperative Group (DBCG) project. The international and national quality control programs that these three laboratories have participated in are reviewed in the present paper. From having a poor degree of inter-laboratory reproducibility in 1981, the inter-laboratory reproducibility has improved due to a collaborative standardization effort. Using the clinical data available through the DBCG program, we have been able to demonstrate that suboptimally performed receptor analyses can influence the conclusions of clinical studies regarding patient prognosis in relation to receptor status.