The distribution and frequency of peptide YY (PYY) cells in 60 gastrointestinal carcinoids and in the nonneoplastic mucosa around the carcinoids were studied by an indirect immunoperoxidase method with anti-PYY serum. Additionally, the endocrine cell type of the PYY cells in appendiceal and rectal carcinoids was assessed by transmission electron microscopy. A few PYY cells were present in specimens of nonneoplastic mucosa from stomach, duodenum, jejunum, ileum, and appendix, with an abundance of these cells in rectal mucosa. Peptide YY cells were found in one of 13 gastric, one of 13 duodenal, one of one jejunal, zero of two ileal, three of 11 appendiceal, and 16 of 20 rectal carcinoids. All but one of the PYY-positive carcinoids were argyrophil carcinoids. Peptide YY cells in the gastric, duodenal, and jujunal carcinoids were present in small numbers. The three PYY-positive carcinoids of the appendix were composed almost totally of PYY cells, whereas those of the rectum generally contained only sporadic PYY cells. The peptide YY cells observed ultrastructurally contained almost round secretory granules (about 160 nm in average diameter), which were most consistent with D1(H) cell type granules with respect to shape and average diameter. This is the first systematic immunohistochemical and ultrastructural study of PYY cells in gastrointestinal carcinoids.