The long-term effects of sigmoidoscopy screening on colorectal cancer (CRC) incidence and mortality in women and men are unclear.

To determine the effectiveness of flexible sigmoidoscopy screening after 15 years of follow-up in women and men.

Randomized controlled trial. (ClinicalTrials.gov: NCT00119912)

Oslo and Telemark County, Norway.

Adults aged 50 to 64 years at baseline without prior CRC.

Screening (between 1999 and 2001) with flexible sigmoidoscopy with and without additional fecal blood testing versus no screening. Participants with positive screening results were offered colonoscopy.

Age-adjusted CRC incidence and mortality stratified by sex.

Of 98 678 persons, 20 552 were randomly assigned to screening and 78 126 to no screening. Adherence rates were 64.7% in women and 61.4% in men. Median follow-up was 14.8 years. The absolute risks for CRC in women were 1.86% in the screening group and 2.05% in the control group (risk difference, −0.19 percentage point [95% CI, −0.49 to 0.11 percentage point]; HR, 0.92 [CI, 0.79 to 1.07]). In men, the corresponding risks were 1.72% and 2.50%, respectively (risk difference, −0.78 percentage point [CI, −1.08 to −0.48 percentage points]; hazard ratio [HR], 0.66 [CI, 0.57 to 0.78]) (P for heterogeneity = 0.004). The absolute risks for death from CRC in women were 0.60% in the screening group and 0.59% in the control group (risk difference, 0.01 percentage point [CI, −0.16 to 0.18 percentage point]; HR, 1.01 [CI, 0.77 to 1.33]). The corresponding risks for death from CRC in men were 0.49% and 0.81%, respectively (risk difference, −0.33 percentage point [CI, −0.49 to −0.16 percentage point]; HR, 0.63 [CI, 0.47 to 0.83]) (P for heterogeneity = 0.014).

Follow-up through national registries.

Offering sigmoidoscopy screening in Norway reduced CRC incidence and mortality in men but had little or no effect in women.

Norwegian government and Norwegian Cancer Society.