• Department of Medicine, Sorlandet Hospital Kristiansand, Kristiansand, Norway.
• Departments of Medicine and Epidemiology, University of Pittsburgh, University of Pittsburgh Cancer Institute, Pittsburgh, PA.
• Centro di Prevenzione Oncologica Piemonte and Città della Salute e della Scienza University Hospital, Turin, Italy.
• Telemark Hospital Skien, Skien, Norway.
• Clinical Effectiveness Research group, Institute of Health and Society, University of Oslo, Oslo, Norway.

OBJECTIVE:  To compare the effectiveness of flexible sigmoidoscopy in screening for colorectal cancer by patient sex and age.

DESIGN:  Pooled analysis of randomised trials (the US Prostate, Lung, Colorectal and Ovarian cancer screening trial (PLCO), the Italian Screening for Colon and Rectum trial (SCORE), and the Norwegian Colorectal Cancer Prevention trial (NORCCAP)).

DATA SOURCES:  Aggregated data were pooled from each randomised trial on incidence of colorectal cancer and mortality stratified by sex, age at screening, and colon subsite (distal v proximal).

ELIGIBILITY CRITERIA FOR SELECTING STUDIES:  Invited individuals aged 55-74 (PLCO), 55-64 (SCORE), and 50-64 (NORCCAP). Individuals were randomised to receive flexible sigmoidoscopy screening once only (SCORE and NORCCAP) or twice (PLCO), or receive usual care (no intervention).

RESULTS:  287 928 individuals were included in the pooled analysis; 115 139 randomised to screening and 172 789 to usual care. Compliance rates were 58%, 63%, and 87% in SCORE, NORCCAP, and PLCO, respectively. Median follow-up was 10.5 to 12.1 years. Screening reduced the incidence of colorectal cancer in men (relative risk 0.76; 95% confidence interval 0.70 to 0.83) and women (0.83; 0.75 to 0.92). No difference in the effect of screening was seen between men younger than 60 and those older than 60. Screening reduced the incidence of colorectal cancer in women younger than 60 (relative risk 0.71; 95% confidence interval 0.59 to 0.84), but not significantly in those aged 60 or older (0.90; 0.80 to 1.02). Colorectal cancer mortality was significantly reduced in both younger and older men, and in women younger than 60. Screening reduced colorectal cancer incidence to a similar extent in the distal colon in men and women, but there was no effect of screening in the proximal colon in older women with a significant interaction between sex and age group (P=0.04).

CONCLUSION:  Flexible sigmoidoscopy is an effective tool for colorectal cancer screening in men and younger women. The benefit is smaller and not statistically significant for women aged over 60; alternative screening methods that more effectively detect proximal tumours should be considered for these women.