Clinical studies have confirmed the efficacy of peptide hormone as a routine tumor marker in the diagnosis, monitoring, and assessment of the prognosis of patients with endocrine tumors. However, in a large number of cancer patients, including those with ectopic hormone-producing tumors, the peptide hormone has limited clinical use as a routine tumor marker because of its poor specificity for tumor tissue and its low concentration in the patient sera. A new analysis product, ProGRP, is a specific and reliable serum tumor marker for small cell lung carcinoma (SCLC). It is useful not only in the evaluation of prognosis but also in the detection of SCLC at an early stage. On the other hand, recently, the RET proto-oncogene has been identified as a gene responsible for multiple endocrine neoplasia (MEN) syndromes: MEN 2A and MEN 2B. In the future, serum peptide hormone markers and molecular genetic markers may be useful in the diagnosis of endocrine tumors.