BACKGROUND. A prevalence peak is expected in breast cancer incidence when mammography screening begins, but afterward the incidence still may be elevated compared with prescreening levels. It is important to determine whether this is due to overdiagnosis (ie, the detection of asymptomatic disease that would otherwise not have arisen clinically). In the current study, the authors examined breast cancer incidence after the introduction of mammography screening in Denmark. METHODS. Denmark has 2 regional screening programs targeting women ages 50 years to 69 years. The programs were initiated in 1991 and 1993, respectively. No screening takes place in the 13 other Danish regions. Data regarding incident breast cancers detected between 1979 and 2001 were retrieved from the Danish Cancer Registry for each screening region and for the rest of Denmark, and time trends in rates for women ages 50 years to 69 years were compared. From 1 program, individual screening data were used to analyze breast cancer incidence in women who were never screened, those who were screened for the first time, or those who previously were screened. RESULTS. The incidence of breast cancer was found to have increased regardless of screening. In the screening regions, a marked prevalence peak was observed, and the incidence hereafter was compatible with the level indicated by the 95% confidence limits for the regression curves for the rates in the prescreening period, taking into account the artificial ageing in the program, the influx of newcomers, and variations in the data. Women who had undergone previous screening were found to have the same incidence of breast cancer as women who were never screened. CONCLUSiONS. The data from the current study do not provide evidence of overdiagnosis of invasive breast cancer in the 2 Danish screening programs or, if overdiagnosis was found to occur, it was only of limited magnitude.