Gonadotropin-releasing hormone (GnRH) 1 and 2 and luteinizing hormone (LH) receptors have been described in the gastrointestinal tract. We have previously demonstrated antibodies in serum against GnRH1 in patients with gastrointestinal dysfunction and diabetes mellitus, and antibodies against GnRH receptor, LH, and LH receptor in patients with infertility. The aim of this study was to search for the expression of serum antibodies against GnRH1 with an improved enzyme-linked immune sorbent assay (ELISA), and antibodies against progonadoliberin-2, GnRH2, GnRH receptor, LH, and LH receptor with newly developed ELISAs, in patients with gastrointestinal dysfunction or diabetes mellitus. Healthy blood donors served as controls. Medical records were scrutinized. Our conclusion was that IgM antibodies against GnRH1, progonadoliberin-2, and/or GnRH receptors were more prevalent in patients with functional gastrointestinal disorders, gastrointestinal dysmotility, and/or diabetes mellitus, whereas IgG antibodies against these peptides, and LH- and LH receptor antibodies, were expressed in the same magnitude as in controls.