Cytokine gene polymorphisms modify expression and their circulating protein levels reflect inflammatory response. Chronic inflammation plays a key role in the pathogenesis of colorectal neoplasia (CRN) associated with inflammatory bowel disease, but it is not clear whether inflammation is a cause or an effect of tumours in sporadic CRN. We therefore investigated the association of cytokine gene polymorphisms and circulating cytokine levels on the risk of CRN in Northeast Scotland, which has a high incidence of CRN. We recruited two groups of patients from a screening colonoscopy cohort, either preprocedure or 3–24 months postprocedure. Participants with CRN were compared with participants with no evidence of CRN (controls). Blood-derived DNA was used to genotype polymorphisms in IL1B, IL1-RN, IL6, IL8, IL10, PTGS2 and TNFA genes. Circulating levels of high-sensitivity C-reactive protein and six cytokines [interleukin-1β (IL-1β), IL-4, IL-6, IL-8, IL-10 and tumour necrosis factor-α (TNF-α)] were measured. To examine the effect of CRN resection on marker levels, we used propensity score matching. A total of 884 patients were eligible for analysis, including 388 CRN cases and 496 controls. Cases were older (mean age 64 vs. 62 years, P<0.01) and more likely to be men (67 vs. 55%, P<0.001). Controls were more likely to be regular users of NSAID (P<0.0001). Compared with homozygous carriage of respective common alleles, proinflammatory CC genotypes of IL1B-31C>T [odds ratio (OR) (95% confidence interval (CI) 1.68 (1.03–2.73)] and PTGS2-765 C>G [OR (95% CI) 2.97 (1.05–8.46)] were each associated with an increased risk of CRN. Conversely, carriage of the A allele of IL8-251A>T was associated with a lower risk of CRN compared with the TT genotype [ORs (95% CI) 0.60 (0.41–0.86) for heterozygous, 0.88 (0.57–1.37) for homozygous, and 0.68 (0.48–0.95) for heterozygous and homozygous combined]. Compared with postprocedure cases, IL8, TNF-α and C-reactive protein levels were significantly higher in preprocedure cases, but IL4 and IL10 protein levels were significantly lower. Proinflammatory cytokine gene polymorphisms in IL1B-31 and PTGS2-765 increase the risk of developing CRN. Levels of proinflammatory IL8, TNF-α and C-reactive protein markers are significantly higher while CRN is in situ. Along with the NSAID findings, these data point to inflammation as an underlying pathogenetic mechanism in CRN.