The paraneoplastic endocrine syndromes ("ectopic" or "inappropriate" hormone production) comprise a wide array of symptom complexes associated with malignant or less commonly benign neoplasms. Most of the syndromes are associated with the production of peptide hormones, which, in some instances, have autocrine stimulatory effects. Hypercalcemia, the most common paraneoplastic endocrine syndrome, may be due to the systemic release of parathyroid hormone-related protein (PTHrP), factors that may be produced locally (cytokines), or by a combination of these mechanisms. A spectrum of other syndromes may be related to the production of specific hormones or growth factors, including insulin-like growth factor and fibroblast growth factor 23. Molecular mechanisms responsible for the development of these syndromes are poorly understood. Mutational events not only may initiate neoplastic transformation but may also lead to the activation (re-expression) of genes responsible for hormone production. Additionally, epigenetic events such as methylation may also be responsible for the development of these syndromes. It is likely that a multiplicity of genetic and epigenetic events may contribute to the development of paraneoplastic endocrine syndromes.