The insulin-like growth factor II (IGF-II) is a small protein implicated in fetal growth and development. It may play a role in the neoplastic process. The IGF-II gene is located on the short arm of chromosome II near insulin and c-Ha-ras I genes. Three distinct promoters control the transcription of this gene, leading to different IGF-II mRNA species. We have analyzed 21 human colorectal tumors and found overexpression of IGF-II in 6 of them (30%). When compared with expression in normal adjacent tissues, IGF-II mRNA increase in these tumors was either moderate (2- to 15-fold) or very marked (200- to 800-fold). In situ hybridization experiments confirmed that high IGF-II mRNA amounts were localized in cancer cells of the tumors overexpressing the IGF-II gene. In addition, DNA analysis revealed a structural modification of one IGF-II locus in one tumor characterized by very high IGF-II mRNA.