Germ-line mutations of the RET proto-oncogene cause three different cancer syndromes: multiple endocrine neoplasia type 2A (MEN2A), multiple endocrine neoplasia type 2B, and familial medullary thyroid carcinoma (FMTC). The objective of the present study was the clinical and molecular characterization of the first two Greek Cypriot families diagnosed with MEN2A and FMTC. The clinical diagnosis of the probands was based on clinical presentation and supported with laboratory findings (calcitonin and carcinoembryonic antigen tumor marker levels). We screened the RET gene by direct DNA sequencing of exons 10, 11, and 16 using genomic DNA as templates. After identification of the mutation, we also developed the amplification refractory mutation system (ARMS) as an alternative method to direct sequencing for genetic diagnosis of 22 additional individuals from both families. We identified the germ-line missense mutation T --> C of codon 618 of exon 10 (C618R) in the probands of both families. By using ARMS, two members of the MEN2A family and five members of the FMTC family were also found positive for the C618R mutation. These are the first seemingly unrelated families in Cyprus investigated clinically and molecularly in detail and shown to transmit this common RET proto-oncogene mutation.