OBJECTIVE: In IBD, interleukin-23 (IL-23) and its receptor (IL-23R) are implicated in disease initiation and progression. Novel insight into which cells produce IL-23 at the site of inflammation at an early stage of IBD will promote the development of new tools for diagnosis, treatment and patient monitoring. We examined the cellular source of IL-23 in colon tissue of untreated newly diagnosed paediatric patients with IBD. DESIGN: Colon tissues from IBD and non-IBD patients were analysed by quantitative real-time PCR (qPCR), immunofluorescence confocal microscopy and flow cytometry after appropriate sample preparation. Blood samples from IBD and non-IBD patients and healthy controls were analysed using flow cytometry and qPCR. RESULTS: We discovered that tissue-infiltrating neutrophils were the main source of IL-23 in the colon of paediatric patients with IBD, while IL-23(+) human leucocyte antigen-DR(+) or IL-23(+)CD14(+) cells were scarce or non-detectable, respectively. The colonic IL-23(+) neutrophils expressed C-X-C motif (CXC)R1 and CXCR2, receptors for the CXC ligand 8 (CXCL8) chemokine family, and a corresponding CXCR1(+)CXCR2(+)IL-23(+)subpopulation of neutrophils was also identified in the blood of both patients with IBD and healthy individuals. However, CXCL8-family chemokines were only elevated in colon tissue from patients with IBD. CONCLUSIONS: This study provides the first evidence of CXCR1(+)CXCR2(+)IL-23-producing neutrophils that infiltrate and accumulate in inflamed colon tissue of patients with IBD. Thus, this novel source of IL-23 may play a key role in disease progression and will be important to take into consideration in the development of future strategies to monitor, treat and prevent IBD. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/