Screening programs for breast cancer are widely used to reduce the impact of breast cancer in populations. For example, the South Australian Breast X-ray Service, BreastScreen SA, established in 1989, is a participant in the National Program of Early Detection of Breast Cancer. BreastScreen SA has collected information on both screening-detected and interval or self-reported cases, which enables the estimation of various important attributes of the screening mechanism. In this paper, a tailored model is fitted to the BreastScreen SA data. The probabilities that the screening detects a tumour of a given size and that an individual reports a tumour by a specified size in the absence of screening are estimated. Estimates of the distribution of sizes detected in the absence of screening, and at the first two screenings, are also given.