OBJECTIVE: The evidence for the effectiveness of screening is strongest for women ages 50 to 69 years; however, there is variation in the target age group for screening programs between different countries. In particular, there is uncertainty over whether women should continue screening once they reach age 70. We therefore investigated incidence rates for invasive and in situ breast cancer by age as well as prognostic features of tumors within a screening program. METHODS: We studied 474,808 women who attended BreastScreen Victoria from January 1, 1993 to December 31, 2000. Of these women, 5,301 were diagnosed with invasive cancer and 1,127 were diagnosed with ductal carcinoma in situ. We used generalized additive models to model age-incidence rates for invasive cancers and ductal carcinoma in situ separately by users and nonusers of hormone replacement therapy at most recent screen. Nonparametric trends for ordered groups and regression methods were used to investigate trends in size, grade, and nodal involvement for invasive tumors by type of attendance and time since previous negative screen for age group. RESULTS: The incidence of ductal carcinoma in situ among women with a previous negative screen clearly declined after age 70 irrespective of hormone replacement therapy use. At subsequent screen, the age-incidence curve for invasive breast cancer flattened at ages 60 to 75 years and then increased only for women taking hormone replacement therapy. Tumor size at diagnosis declined with age at both first round (P = 0.15) and subsequent round (P = 0.08). The proportion of poorly differentiated tumors also decreased with age, with the smallest proportion of grade III tumors diagnosed in women ages > or = 75 years (P = 0.09 for first screen and P = 0.05 for subsequent screen). The presence of positive nodes at diagnosis declined with age (P < 0.001) for both first and subsequent screening rounds. CONCLUSION: Older age is associated with more favorable prognostic tumor features and a lower incidence of ductal carcinoma in situ among subsequent attenders of screening. When making decisions regarding continuing screening, older women and their physicians should also consider the presence of other comorbid conditions that may mitigate any impact of screening on mortality.