Insulin-like growth factor-II (IGF-II) is a potent mitogen for a variety of cell types and is considered an important regulator of breast cancer growth. In this study, we analyzed IGF-II mRNA and protein expression in a series of 80 cases of invasive breast cancer. Seventy-five cases produced informative results for IGF-II mRNA expression, and were scored on an arbitrary scale. Two cases (2.6%) had no significant IGF-II mRNA expression. 35 cases (46.7%) expressed low levels of IGF-II mRNA, 20 cases (26.7%) moderate IGF-II mRNA, while 18 (24%) expressed high levels of IGF-II message. Generally, IGF-II mRNA was expressed in the smooth muscle walls of blood vessels and ducts, as well as in the stroma tightly adjacent to and surrounding tumor epithelium. IGF-II mRNA content was also directly related to the amount of the stroma within the tumor (p < 0.05). In 10 cases (13.3%) IGF-II mRNA was detected in the stroma of normal lobules. Fifty-six out of 75 were positive for IGF-II immunostaining. Again, protein staining was generally observed in the smooth muscle of both blood vessels and ducts, as well as in the stroma surrounding tumor epithelium. In normal lobules and ducts the IGF-II protein was detected in the myoepithelium. Unequivocal IGF-II protein staining was seen in tumor epithelium in only three cases. The results of our study demonstrate that, in breast cancer, IGF-II mRNA is expressed in the smooth muscle and stromal components in the majority of invasive breast cancers. IGF-II expression correlates positively with the amount of stromal tissue present within a tumor. This suggests that IGF-II may have an important growth regulatory effect on breast tumor epithelium through paracrine pathways.