Paraneoplastic syndromes are a group of clinical manifestations associated with a malignancy, but not directly related to the primary tumor itself or to its metastases. Characteristically, they follow a course parallel to the tumor, resolve with successful treatment of the primary tumor, and tend to recur with its relapse or the onset of metastases. The mechanism by which they occur is not well understood, but may be related to the production of bioactive substances by or in response to the tumor, such as polypeptide hormones, hormone-like peptides, antibodies or immune complexes, cytokines, or growth factors.