IGF-II is a polypeptide growth factor with growth and differentiation promoting activities, involved in human development. We have reported previously IGF-II mRNA and peptide overexpression in primary human colon cancers. Here we show that the IGF-II peptide content is increased in six primary colon cancers compared to the corresponding healthy tissues. The IGF-II transcripts in healthy and cancerous colon tissues were identified by Northern blotting and RT-PCR. Promoters P3 and P4 were active in most tissues. Relaxation of parental imprinting was observed in two tumors and one healthy tissue, without any correlation with the IGF-II transcript levels. Rearrangements of the IGF-II gene in two tumors containing very high amounts of IGF-II mRNA are described. Fragments containing the breakpoints were cloned by the vectorette-PCR strategy. In both tumors, the breakpoints occurred in repetitive sequences. In one tumor (T11), the breakpoint was localized 2 kb downstream the end of exon 9. The second tumor (T18) contains two modified alleles. In one rearranged allele the breakpoint is located in exon 9. The exact position of the breakpoint in the second rearranged allele has not been identified. In future experiments, the correlation between the gene rearrangements and IGF-II mRNA overexpression will be studied.