A number of primary human breast carcinomas exhibit amplification of the chromosome 11 region containing the int-2/fgf-3 proto-oncogene, and progression of breast cancer has been correlated with int-2 amplification or with certain restriction fragment length polymorphisms (RFLPs) of the int-2 gene. Using the polymerase chain reaction (PCR), we obtained the int-2 coding sequences from six primary tumors, four of which exhibited amplification of the int-2 gene and one of which exhibited amplification of the neu gene. The majority of these tumors (five of six) were aggressive, as judged by their early recurrence, metastasis, or both. Nucleotide sequencing of PCR products revealed that previously described BamHI and PstI RFLPs of the int-2 gene, as well as a new polymorphism at position 9154, were located within the intron between the second and third exons. A seventh tumor was used to localize one of the PstI RFLPs 5 bp from the splice-acceptor site of the third exon. However, none of the tumor DNAs analyzed showed differences in the int-2 protein coding regions when compared with normal placenta DNA. These results imply that aggressive human breast cancers encode an unaltered form of the int-2 protein.