We investigated the frequency of rearrangements of the ret and trk proto-oncogenes in Japanese thyroid tumors. DNAs from 38 thyroid papillary carcinomas and 14 follicular adenomas were analyzed by Southern blotting. Rearrangements of the ret and trk proto-oncogenes were detected in one and two papillary carcinomas, respectively, but not in follicular adenomas. Analysis by a reverse transcriptase-polymerase chain reaction method showed that the ret rearrangement-positive tumor contained the PTC/retTPC chimeric transcript, which was reported to be found specifically in thyroid tumors and adenomatous goiter. We also found that rearranged mRNA of the trk proto-oncogene was expressed at high levels in one of two trk rearrangement-positive tumors. Our results indicated that the frequency of rearrangements of these proto-oncogenes in Japanese papillary carcinomas was much lower than that in Italian patients.