Performance measures for the Ontario Breast Screening Program (OBSP) by age group, time period and screening modality from 10 years of breast screening were evaluated. Data were available from routine information collected on 283,962 women aged 50 to 69 screened at 73 screening centres between 1 July 1990 and 31 December 2000. Although, initially, participation in the OBSP was low, this rate increased over time and the majority of women screened returned for subsequent screening. Abnormal call rates increased slightly over the time period, were higher in women aged 50 to 59, and for women with mammographic abnormalities. Detection rates of invasive cancer were higher and prognostic features of cancers were better for women age 60 to 69, and those referred by mammography. Along with the prognostic features of cancers, the benign to malignant surgical ratio and diagnostic interval improved over the time periods and for women aged 60 to 69. Greater proportions of women had shorter diagnostic intervals and were more likely to have a diagnosis of breast cancer after surgery if they were referred by both clinical breast examination and mammography. Although some enhancements of the programme are necessary, the OBSP met or exceeded Canadian targets for most performance measures.