The expression of estrogen (ER) and progesterone (PR) receptors was assayed by steroid binding in a series of 95 malignant breast tumors, for which the analysis of chromosome aberrations was performed and allowed the reconstruction of their chromosomal evolution. It was shown that breast tumors undergo a progressive loss of chromosomes, with occasionally one and rarely two endoreduplications. Chromosome losses were often the consequence of rearrangements, and the rate of rearranged chromosomes, which increases progressively, appeared as a possible indicator of tumor progression. The distribution of ER and PR values in the sample of 95 tumors was compared to that of a larger control series of consecutive cases: 598 for ER and 460 for PR. The similarities of the distributions indicated that the sample of 95 tumors was representative of the general population of breast cancers. The levels of ER and PR expression were very strongly and negatively correlated to the rate of rearranged chromosomes, but not to the modal number of chromosomes. However, when tumors having either undergone endoreduplication or not (greater than 50 or less than 51 chromosomes, respectively) were considered separately, a significant correlation between ER and PR expression and chromosome number was found within each group. Finally, breast cancers were subdivided into 4 stages of cytogenetic evolution, from the least to the most evolved: stage 1: less than or equal to 50 chromosomes, less than 25% rearranged chromosomes; stage 2: greater than 50 chromosomes, less than 25% rearranged chromosomes; stage 3: less than or equal to 50 chromosomes, greater than 25% rearranged chromosomes; stage 4: greater than 50 chromosomes, greater than 25% rearranged chromosomes.(ABSTRACT TRUNCATED AT 250 WORDS)