The appendix is lined by a mucosa which has many neuroendocrine cells containing serotonin. Local release of serotonin can act as a mediator of inflammation. In this study we explored the serotonin content of the neuroendocrine cells of the appendixes removed for clinical diagnosis of appendicitis. Appendix specimens were divided into three groups: Acute appendicitis (AA), non-appendicitis (NA), and follicular hyperplasia (FH). Normal appendix specimens from patients undergoing elective abdominal surgery were used as the control group (NL). All sections were exposed to proteinase K, incubated with anti-serotonin, chromogranin A, and synaptophysin antibodies, and treated with the LSAB kit. Polygonal cells were seen within the crypt epithelium (enterochromaffin cell, EC) and within the lamina propria (subepithelial neuroendocrine cell, SNC). In AA, only 16 cases (64%) showed serotonin staining in non-destructed glands. There was a significant reduction in the number of ECs in AA compared to the FH (96%), NA (100%) and NL (100%) groups (P<0.001). Chromogranin and synaptophysin immunostaining also showed a significant reduction in the number of ECs in AA compared with the other three groups (P<0.001). SNC serotonin reactivity was lower in the AA group compared with the other groups (p<0.001). The inflamed appendix is markedly depleted of serotonin in the epithelium and lamina propria. Local serotonin release from ECs and SNCs in the appendix may act as an inflammatory mediator in appendicitis and is likely to be the source of raised blood serotonin in AA.