We examined RET protooncogene mutations in sporadic medullary thyroid carcinoma (MTC), using polymerase chain reaction (PCR)-based sequencing. DNA was extracted from tumor tissue and peripheral blood leukocytes of seven unrelated individuals with apparently sporadic MTC. Oligonucleotide primers were selected to amplify exons 10, 11, 13, 15, and 16 of the RET protooncogene, to examine the sequences of codons 609, 611, 618, and 620 of exon 10, codon 634 of exon 11, codon 768 of exon 13, codon 883 of exon 15, and codon 918 of exon 16. Direct DNA sequencing from PCR products was then performed. The results showed that one patient had a somatic mutation at codon 918 (ATG-->ACG), causing a Met-->Thr substitution. One patient had a de novo germline mutation at codon 634 (TGC-->CGC), causing a Cys-->Arg substitution. Another patient had a germline mutation at codon 634 (TGC-->TTC), causing a Cys-->Phe substitution. In the remaining four cases, no RET mutations were found. Unexpectedly, two offspring of the patient (a female) with a germline mutation at codon 634 (TGC-->TTC) harbored homozygous alleles for the mutation; because the father did not carry this mutation, the other affected allele was suspected to have resulted from a de novo germline mutation of paternal origin. One of these offspring was subsequently diagnosed as having MTC. Our findings suggest that all patients with apparently sporadic MTC should be screened for the RET protooncogene by molecular analysis to detect occult or de novo multiple endocrine neoplasia 2 (MEN 2) or familial MTC. This would allow early treatment of affected family members.