Retinoids modulate gene activity, cell growth, and differentiation by binding to a series of nuclear receptors, i.e., retinoic acid receptors (RARs) or retinoid X receptors. Retinoic acid (RA) inhibition of estrogen receptor (ER)-positive breast carcinoma seems to be mediated through RARα. Estrogens upregulate RARα in ER-positive breast carcinoma cell lines. In this study we examined RARα expression in the ER-positive MCF7 and ER-negative MDA-MB-231 human breast carcinoma cell lines as well as in 10 ER-negative and 9 ER-positive infiltrating ductal breast carcinoma specimens using immunohistochemistry and quantitation by image cytometry. MCF7 cells expressed twofold higher levels of RARα protein than MDA-MB-231 cells. RARα expression, as detected by immunostaining and quantitated by image cytometry, was upregulated in these cells by estradiol. ER-positive breast carcinoma specimens also exhibited approximately twofold higher RARα levels than their ER-negative counterparts. Thus, RARα expression is significantly elevated in ER-positive breast tumors as assessed by detection and quantitation using immunohisotchemical staining and image cytometry, respectively. Whether the decrease in RARα protein levels and loss of RA-mediated growth inhibition in ER-negative tumor plays a role in the increased metastatic potential of ER-negative tumors remains to be determined.