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In addition to TNM stage there are adverse tumour and host factors, such as venous invasion and the presence of an elevated systemic inflammatory response (SIR), that influence the outcome in colorectal cancer. The present study aimed to examine how these factors varied in screen-detected (SD) and nonscreen-detected (NSD) tumours.Prospectively maintained databases of the prevalence round of a biennial population faecal occult blood test screening programme and a regional cancer audit database were analysed. Interval cancers (INT) were defined as cancers identified within 2 years of a negative screening test.Of the 395 097 people invited, 204 535 (52%) responded, 6159 (3%) tested positive and 421 (9%) had cancer detected. A further 708 NSD patients were identified [468 (65%) nonresponders, 182 (25%) INT cancers and 58 (10%) who did not attend or did not have cancer diagnosed at colonoscopy]. Comparing SD and NSD patients, SD patients were more likely to be male, and have a tumour with a lower TNM stage (both P < 0.05). On stage-by-stage analysis, SD patients had less evidence of an elevated SIR (P < 0.05). Both the presence of venous invasion (P = 0.761) and an elevated SIR (P = 0.059) were similar in those with INT cancers and in those that arose in nonresponders.Independent of TNM stage, SD tumours have more favourable host prognostic factors than NSD tumours. There is no evidence that INT cancers are biologically more aggressive than those that develop in the rest of the population and are hence likely to be due to limitations of screening in its current format.