Objective: To assess the impact of the National Bowel Cancer Screening Program (NBCSP) in South Australia. Design, setting and participants: A cohort comparison of colorectal cancer (CRC) patient data from the NBCSP register and the South Australian Cancer Registry. Patient records of those invited to take part in screening through the NBCSP, those who participated in the program, and those with positive test results were compared with those of the rest of the study population (excluding the group of interest) on an intention-to-screen basis. Main outcome measure: Stage of CRC at diagnosis as a surrogate marker for effect on CRC mortality. Results: Of 3481 eligible patients, 221 had been invited to the NBCSP. Invitees were more likely to have stage A lesions compared with all other patients (34.8% versus 19.2%; P < 0.001), and half as likely to have stage D CRC (5.4% versus 12.4%; P< 0.001). A further shift towards earlier stage was seen in those who participated in screening and those with positive test results compared with all other patients (38.8% stage A and 3.0% stage D in screening participants versus 19.3% stage A and 12.4% stage D in all other patients; and 39.7% stage A and 2.6% stage D in those with positive test results versus 19.3% stage A and 12.4% stage D in all other patients; P < 0.001). Conclusions: CRCs were diagnosed at a significantly earlier stage in people invited to the NBCSP compared with those who were not invited, regardless of participation status or test result. The NBCSP should lead to reductions in CRC mortality in Australia.