Background: Dietary fiber has been associated with a reduced riskof colorectal cancer. However, it remains unclear at which stage inthe carcinogenic pathway fiber may act or which food sources ofdietary fiber may be most beneficial against colorectal cancer development.Objective: The objective was to prospectively evaluate the associationbetween dietary fiber intake and the risk of incidentand recurrent colorectal adenoma and incident colorectalcancer.Design: Study participants were identified from the interventionarm of the Prostate, Lung, Colorectal, and Ovarian CancerScreening Trial. Participants received flexible sigmoidoscopy atbaseline and 3 or 5 y after. Dietary fiber intake was measured byusing a self-reported dietary questionnaire. The colorectal cancer,incident adenoma, and recurrent adenoma analyses werebased on 57,774, 16,980, and 1667 participants, respectively.Unconditional logistic regression was used to assess the risk ofincident and recurrent adenoma, and Cox proportional hazardsmodels were used to assess the risk of colorectal cancer acrosscategories of dietary fiber intake, with adjustment for potentialconfounders.Results: Elevated total dietary fiber intake was associated witha significantly reduced risk of incident distal colorectal adenoma(ORhighest vs. lowest tertile of intake: 0.76; 95% CI: 0.63, 0.91; P-trend =0.003) but not recurrent adenoma (P-trend = 0.67). Although theassociation was not statistically significant for colorectal canceroverall (HR: 0.85; 95% CI: 0.70, 1.03; P-trend = 0.10), a reducedrisk of distal colon cancer was observed with increased total fiberintake (HR: 0.62; 95% CI: 0.41, 0.94; P-trend = 0.03). Protectiveassociations were most notable for fiber originating from cereals orfruit.Conclusions: This large, prospective study within a population-basedscreening trial suggests that individuals consuming the highest intakesof dietary fiber have reduced risks of incident colorectal adenoma anddistal colon cancer and that this effect of dietary fiber, particularly fromcereals and fruit, may begin early in colorectal carcinogenesis. This trial was registered at clinical trials.gov as NCT01696981.