Detection and removal of adenomas reduces colorectal cancer (CRC) risk. The impact of adenoma detection rates (ADRs) on long-term CRC incidence and mortality is unknown. We investigated this using data from the UK Flexible Sigmoidoscopy Screening Trial (UKFSST).Of 167,882 UKFSST participants, 40,085 were in the intervention arm and underwent flexible sigmoidoscopy screening at 13 trial centres. Median follow-up was 17 years. At each centre, one endoscopist performed most flexible sigmoidoscopies. Multivariable logistic regression was used to classify centres into high-, intermediate-, and low-detector groups based on their main endoscopist's ADR. We calculated incidence and mortality of distal and all-site CRC, and estimated hazard ratios (HRs) with 95% confidence intervals (CIs) using Cox regression.Five, four, and four centres, respectively, were classified into the high-detector, intermediate-detector, and low-detector groups. Average ADRs in each respective group were 15%, 12%, and 9%. Distal CRC incidence and mortality were reduced among those screened compared to controls in all groups, and effects of screening varied significantly by detector ranking, with larger reductions in incidence and mortality seen in the high-detector (incidence: HR=0·34, 0·27-0·42; mortality: HR=0·22, 0·13-0·37) than low-detector group (incidence: HR=0·55, 0·44-0·68; mortality: HR=0·54, 0·34-0·86). Similar results were observed for all-site CRC, with larger effects seen in the high-detector (incidence: HR=0·58, 95%CI 0·50-0·67; mortality: HR=0·52, 0·39-0·69) than low-detector group (incidence: HR=0·72, 0·61-0·85; mortality: HR=0·68, 0·51-0·92), although the heterogeneity was not statistically significant.Higher ADRs at screening provide greater long-term protection against CRC incidence and mortality.