The susceptibility gene for hereditary Medullary Thyroid Carcinoma (MTC) is the RET proto-oncogene. The aim of this study was to evaluate the prevalence of common germline RET mutations in exons 10 and 11 among Iranian MTC patients. Fifty-seven non-related MTC patients were examined in this study (Females: Males =1.2:1.0, Mean age = 40.0 +/- 11.5 years) and the existence of mutations was assessed through the PCR-RFLP technique. The only Multiple Endocrine Neoplasia type 2A (MEN2A) patient displayed a C634W mutation in exon 11. Among 53 apparently sporadic MTC patients, one patient showed a C620R mutation in exon 10 and two other patients displayed C624Y mutations in exon 11 of RET proto-oncogene. Neither the only Multiple Endocrine Neoplasia type 2B (MEN2B) patient nor two Familial MTC patients was found to carry germline mutations in exons 10 and 11. This study reports, for the first time, the prevalence of common RET mutations among Iranian, apparently sporadic MTC patients, underlining the critical importance of screening for RET mutations in such patients.