Messenger RNA expression of retinoic acid receptors (RARalpha, RARbeta and RARgamma) and retinoid X receptors (RXRalpha, RXRbeta and RXRgamma) was examined using reverse transcription-polymerase chain reaction in 42 papillary thyroid carcinomas (PTCs). A loss of mRNA expression was observed in 18 cases of the 42 PTCs, including three cases for RARalpha, 14 cases for RARbeta, six cases for RXRalpha and five cases for RXRbeta. The expressions of RARgamma and RXRgamma were found in all 42 PTCs. Based on Ki 67/MIB1 labeling index (LI), the 42 PTCs were classified into Group A (20 cases; LI = 0-2%), Group B (17 cases; LI = 2-5%) and Group C (5 cases; LI > 5%). The PTCs of groups B and C showed solid, trabecular or scirrhous arrangements, infiltrative growth, loss of cellular polarity and cohesiveness more frequently, but capsulated growth pattern less frequently, when compared with PTCs of Group A. They also showed more frequent extrathyroidal extension than Group A. However, no significant differences were identified in sex, age, nodal status and tumor size. Loss of expression for one or more retinoid receptors frequently occurred in groups B and C. These results suggest that the loss of retinoid receptors might occur during the loss of differentiation and tumor progression of PTC.