ABSTRACT Objectives: To determine the response to colorectal cancer (CRC) screening by colonoscopy, through direct invitation or through invitation by general practitioners. Design and setting: Two-way comparison of randomised population sampling versus cluster sampling of a representative general practice population in the Australian Capital Territory, May 2002 to January 2004. Intervention: Invitation to screen, assessment for eligibility, interview, and colonoscopy. Subjects: 881 subjects aged 55-74 years were invited to screen: 520 from the electoral roll (ER) sample and 361 from the general practice (GP) cluster sample. Main outcome measures: Response rate, participation rate, and rate of adenomatous polyps in the screened group. Results: Participation was similar in the ER arm (35.1%; 95% CI, 30.2%-40.3%) and the GP arm (40.1%; 95% CI, 29.2%-51.0%) after correcting for ineligibility, which was higher in the ER arm. Superior eligibility in the GP arm was offset by the labour of manual record review. Response rates after two invitations were similar for the two groups (ER arm: 78.8%; 95% CI, 75.1%-82.1%; GP arm: 81.7%; 95% CI, 73.8%-89.6%). Overall, 53.4% ineligibility arose from having a colonoscopy in the past 10 years (ER arm, 98/178; GP arm, 42/84). Of 231 colonoscopies performed, 229 were complete, with 32% of subjects screened having adenomatous polyps. Conclusions: Colonoscopy-based CRC screening yields similar response and participation rates with either random population sampling or general practice cluster sampling, with population sampling through the electoral roll providing greater ease of MJA 2004; 181: 423-427 recruitment
