The age-adjusted incidence of breast cancer among Wisconsin women age 40 and older has increased by almost one third since the early 1980s. To estimate what portion of the observed increase in breast cancer incidence is due to mammography screening, we developed a model of the lead-time effect of this screening test and its impact on incidence. The model incorporates annual age-specific information including 1) the expected number of cases, 2) the rate of screening, 3) the detection ratio of screening mammography, and 4) the lead time of screening mammography. For women 40 years of age and older, the model predicts a 25% increase in incidence, compared with an observed increase of 28%. Overall, mammography screening explains 74% of the difference between the expected and observed number of cases over the study period. A greater portion of the increase in incidence among postmenopausal women is attributed to mammography screening than among younger women. The increase in the use of mammography appears to account for most but not all of the increased incidence of breast cancer in Wisconsin.