Insulin receptor (IR) content is increased in most human breast carcinomas when compared to normal breast tissue. In the present study we investigated IR gene copy number by using both conventional DNA analysis (slot blot) and fluorescence in situ hybridization (FISH). Cultured human breast cell lines and primary breast carcinoma specimens were analyzed. In 6 breast cell lines in culture both techniques gave similar results: the relative IR copy number determined by FISH strongly correlated with slot blot results (r = 0.831), even if probes for different reference loci were used in the 2 methods. We find that in human breast cancer IR gene amplification is a sporadic event. It occurred in 1/5 cultured breast cancer cell lines (MDA-MB 231) and in 8/93 (8.6%) breast cancer specimens. In contrast an increased copy number of the entire chromosome 19 (which contains IR gene) was frequently observed in both breast cancer cell lines (100%) and breast cancer specimens (45%). When present, IR gene amplification always occurred at low level. These data indicate that IR gene amplification is an uncommon event in human breast carcinomas and that mechanisms other than gene amplification are responsible for IR protein overexpression in most human breast cancers.