The ret/PTC oncogene, a rearranged form of the ret proto-oncogene (c-ret), has been detected specifically in a minority of papillary thyroid carcinomas. Three forms of the ret/PTC oncogene have been identified; the two most common forms, ret/PTC-1 and ret/PTC-3, both result from a paracentric inversion of the long arm of chromosome 10. In this study, we have successfully amplified the chimeric introns resulting from these inversions, ranging from 1.4 to 10 kb, from four of five tumors known to contain the ret/PTC-1 oncogene (where c-ret rearranges with the H4 gene), and from 1/1 tumor containing the ret/PTC-3 oncogene (where c-ret rearranges with the ele1 gene). We localized the breakpoints within the chimeric introns using nested PCR, and determined the exact nucleotide sequence at the breakpoint for each tumor. Our results indicate that the breakpoints in c-ret occur at sites distributed across intron 11, while breaks in H4 intron 1 appear to occur more frequently at the 5′-end of the intron. Interestingly, in all tumors that we investigated, the breakpoints occurred at sites of two or three nucleotide matches between the contributing germline sequences. In summary, we describe a simple, convenient way to investigate the ret/PTC breakpoints, and have revealed several common features of the breakpoints which warrant further investigation.