In papillary thyroid carcinogenesis, the constitutively activated mitogen-activated protein (MAP) kinase signaling pathway caused by a genetic alteration such as RET/PTC rearrangement or mutation of RAS and BRAF genes, is thought to be a major early event. Among these, the recently identified BRAFV600E mutation has been found at high frequency in adult patients with papillary thyroid carcinoma (PTC). However, the association between this mutation and radiation exposure in adult PTC is still unknown. In this study, we examined the BRAFV600E mutation in 64 PTCs among adult atomic bomb survivors in Hiroshima, Japan, comprising 17 nonexposed (0 mGy) and 47 exposed patients who developed the carcinoma after the bombing, and assessed the association of BRAFV600E mutation with clinico-pathological and epidemiological variables. The median radiation dose in PTCs with the BRAFV600E mutation was significantly lower than that without the mutation (18.5 vs.156.9 mGy, Wilcoxon rank-sum test, P = 0.022). A significant difference was found in the median latency period (years elapsed from atomic bombing to diagnosis) between exposed patients with and without BRAFV600E mutation (29 vs. 21 yr, Wilcoxon rank-sum test, P = 0.014). These findings were further confirmed by logistic regression analysis with BRAFV600E mutation status as a dependent variable and taking into account possible interactions between the variables. We found that the log-transformed radiation dose and latency period were independently associated with the BRAFV600E mutation (P = 0.039 and P = 0.010, respectively). These results suggest that involvement of BRAF mutation in thyroid carcinogenesis in exposed people may differ from that in the nonexposed people. © 2006 Wiley-Liss, Inc.